Using headphone amp outputs simultaneously

Just curious what the rule of thumb is on this? I’ve never taken the chance of doing it myself, but I know some headphone amps (like most Stax) are designed to allow more than one set of headphones connected simultaneously. My thinking is that it’s not good to do, otherwise I wouldn’t see so many elaborate means of A/Bing headphones on youtube. I have no real need of doing this, but I have 2x ZMF headphones coming soon and they apparently benefit from a good amount of burn in. I was thinking this would speed up the process, but I was still planning on running 2 separate amps while at work. It’s more of a curiosity than anything else

For some less powerful amps, it can hinder performance.
In a longer explanation, not all amps are built to handle driving multiple headphones to their best extent, and thus will result in lower volume being produced.

I figured that was the main reason, but would it be safe to use the THX789 to burn in multiple headphones with pink noise? The thing that got me thinking about it was the description for the Studio 6 tube amp that specifically states ideal for shared listening sessions. I know it has substantially more power than the 789, but it got me questioning if anyone should do it if the amp wasn’t explicitly stated as being capable out of fear of damaging the amp or the headphones.

Here is the Studio 6 if anyone was curious:

Now that one, I can’t quite answer. I don’t own a 789, and can’t test it. If you already have a couple headphones that you can give it a test run with, then feel free to try it.

For most amps it will hinder performance unless it’s made to have multiple headphones running off of it at once