I will put some thought into a save-ish way to do what you are looking to achieve here.
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Might be worth sending an email to Singxer for some pointers too, or via ASR which one of their engineers participates there (under the handle tom.chen).
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There is also an option to run the balanced outputs of the amps, 4.4, or 4 pin XLR, with a custom cable termination to TRS or mini XLR into the earcups that take those terminations. Technically this would keep the signal output balanced. In any case, balanced or not, each amp will only see the ohm load of a single driver. If its above 32ohms it should be fine I would think. Of course 150-300ohms might be more ideal.
Okay, some back-of-napkin thoughts:
The easiest is putting Stereo-to-Mono and mono-Y-splitters behind the amps.
Now the harder part:
This is an important check! There may be DC-Offsets ranging somewhere between a few mV up to single digit Volts between devices or channels. Offset voltage divided by output impedance should give a ball park figure of power lost to the amp fighting itself (or its neighbour).
The basic dangerzone setup:
I STRONGLY advise against doing it this way!
To lower the risk of magic smoke release, I propose using some 4 Ω resistors (≥ 1W each) in all potential current paths:
This way, the lowest impedance the amps should ever see is 8 Ω (ignoring LCR-properties of the resistors used). Putting a “mute switch” in the pink and blue paths leading to the out jack to temporarily disconnect both amps during plug insertion is also advised.
Imaginary Q&A:
“My multimeter says 0” - Is it a hardware-store-check-out-special? Because it may be off by a couple percent and some counts.
“I volume match with my multimeter” - What frequency range does it cover and is it true RMS?
“But my Dollarstore meter…” - “NO! It is NOT accurate!”
I don’t really understand any of that. I’m not an electronics engineer. I just know outputs to inputs, red to red, white to white.
Instead of buying parts & building line level high pass boxes with a subwoofer bypass output, I just bought a Tannoy Definition Subwoofer that has balanced XLR high pass built in.
But you do realize that what you want Is not just simple signal input / output or red to red and white to white? You really should stop before something bad happens.
If you need more power, just buy a more powerful headphone amp. Keep it simple and in the Red Red - White White levels.
1 x Preamp → 1 x AMP and that’s it.
E: Or maybe buy a VOLOT Dual Mono Class-A Headphone Amplifier with Pre-Amp?
This probably does everything you would need right only in single device?
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I would describe myself as an “ambitious hobbyist”.
Thing is, doing “red to red” as you say, may result in two dead.
This.
Setting up what you are after is probably feasible, requires some careful planning and testing before hand though.
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I know. Thinking it out, all I need to figure out is, are there quality headphones that use a balanced TRS or mini XLR input in each cup? Then all I need are 2 custom cables. Hello Hart Audio. The amps will not know they’re not pushing a stereo signal, particularly if the ohm load of the respective driver is equal to or higher than 32ohms. Balanced will be the way to go I’m sure.
Currently the single ended sounds exceptional. Granted, half of each amp is idle. Last night my S599SE sounded very muffled after listening to 600 ohm DT880 & 250ohm DT1990 Pro. Tonight as the 599s were already plugged in, they sound fantastic. Listening to Essential Billy Joel, & Chicago 17.
I mean it was just one option.
Bryston BHA-1 probably does the same. orderable atleast from 3 different stores = in stock.
Has a Balanced XLR Pair (3 Pin Left/Right)
So there probably are even more options than these 2 if you just search for them.
But since you have the amps. Lets focus on headphones.
Yes!
Pick a pair of quality headphones and then just add little DIY on top. 
Red to red and white to white and black to black. Things you do know.
Like in here, someone did.
But for this project probably Audeze would have good options as well since they have 4-pin XLR’s on both cups. Probably just a red-red and white-white thing after all. Who knows.
Keep us posted how the project continues.
I’m almost 100% sure only 2 of those pins are used per side. Having rewired a couple pairs of 600ohm beyers myself to a single 4 pin mini-XLR, there was only 4 wires, 2 for each side. I think only 2 pins are used on each side of the Audeze plugs as well.
I come from a musician’s background pertaining to audio electronics, so I know what cable goes where & why. XLR for microphones, TRS for DI outputs that may have a hiZ/SE/TS input/electronic keyboard outputs/electronic drum outputs; of which all plug into the mix console preamps. A DI can be used for any electronic instrument that doesn’t have a balanced output. Often bass guitars will use a DI. The PA amps used either TRS or TS as speaker wires to the PA speakers. I forget which.
I have some experience in car audio also. I know each speaker has a +/positive/hot & a -/negative/ground connection, but that’s about it.
I know most amplifiers increase power as the ohm load lowers & some amps double power as the ohm load halves. Those tend to be expensive. I’ve never owned an amp that can double power output from 8ohms to 4ohms & so on. Some amps are a total mystery to me. Those amps that cut power as the ohm load lowers. Whats even more weird is they tend to cost a lot more.
I had a point. Where am I? Oh, I’m not a DIY person. I have an understanding of some things. That’s about it.
If a headphone is really balanced I would think it must have either a 3 pin XLR per side, or TRS, as a balanced connection requires a positive, a negative, & a neutral ground, hence the 3 pins of XLR & the Tip/Ring/Sleeve of TRS? Therefore 4 pin stereo XLR can’t really be balanced can it?
Lots of life getting in the way. No further developments as yet. I’m about to go ride my motorcycle.
I do have an observation. My DT150 have a 6 pin connection. This would indicate that they are truely balanced IMO, but they only have a TRS termination. Sucks. I’m waiting on a cable. They shipped without one, or it was lost in trasit.
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For Sure.
But if you are connecting two balanced amps in one headphone per two cups.
How many wires you would need, using a XLR connectors? 
Im just curious how the project ends now and not the answer (MazeFrame answer above)
DT150 cable showed up while I was riding my motorcycle.
The DT150s sound great & they’re super light. 250ohm closed back, old school, but not clanky or squeaky.
DT150 are not as light as 599SE. The DT150 are also giving me hot ears compared to the 599SE. My ears are fully surrounded/enclosed by the DT150 earcups.
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Balanced signals require 2 pins: Signal and inverse Signal (= same signal, but upside down).
The third pin (= shield) is kind off optional.
I had them on my wishlist for a while. Maybe a purchse for christmas?
Since current setup mentioned.
Any new thoughts how to proceed with the plan in First post?
To actually make it work or more power and have Mono amped and maybe even fully balanced all the way.
Well, this has been pretty interesting so far. Somehow in all the back and forth I forgot something… why is this being tried again? I guess I have lost sight of what the potential benefit of this setup would be over, say, just buying a high-quality amp and being done with it? If it’s just for the sake of running a unique experiment, OK I guess. I will say that I feel like that is a pretty expensive experiment! But hey, it’s all good. Best of luck with it. I am anxious to see what the final result is.
Just like there is no need to modify headphones. Just buy good headphones. Your argument makes a lot of sense.
People putting shade on a Singxer SA1, have never heard it, or they’re so deaf to treble they need an amp with a hyper treble boost or V shape response, which the Singxer SA1 is not.
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I tend to agree on the SA-1. I feel like the SA-1 is a fine amp. I happen to love mine. Are there better amps out there? Of course there are. But the price to performance aspect of this piece is kinda hard to look past. All I can say is that it’s workin for me! If/when it goes, I’ll make the next jump up which I am trying to save up for now. I am certainly in no hurry.
As for the project, I am in no way trying to bash the effort. In fact, I think it’s cool that you are stepping up and trying something different. If it works, fantastic! If it doesn’t, at least you gave it a shot. Again, I am anxious to see what the final result will be.