How to connect 2 Bluetooth headphones to Apple TV 4K?

I'm not able to see where Apple state you can use 2 BT headsets, you appear to be saying you have seen this claim, perhaps you could point us to where you have seen it. It doesn't make any such claim here...

Use Bluetooth accessories with your Apple TV 4K or Apple TV (4th generation) - Apple Support

Also, whilst I haven't read up exactly what the BT 5 standard includes for, it strikes me that if it's supposed to support 2 connections as you state, then it appears to do that admirably for me as I have previously successfully connected a BT headset at the same time as my Siri BT remote is connected.

Bluetooth is a one-on-one connection protocol by design, so what you want is actually what it isn’t supposed to do.

There are aftermarket dual bluetooth transmitters (e.g. Coulax, Avantree) that will plug in to your tv or receiver (thus not connect directly to the Apple TV) and support two bluetooth headphones. Check for low latency specification and the dual headphone support.

I stand corrected. There is BT5 in the 5th gen.

Back to the headphones: With Bluetooth 5, nothing has changed to Bluetooth Classic; the changes are with Bluetooth Low Energy. The connecting bluetooth devices (headphones, speakers) have to be BT5 LE as well for the new features to be even possible. That likely means buying new ones to that spec. And the OS has to support this feature to provide a configuration for this feature.

(It appears Samsung has made its own adjustments for dual audio, it is not doing it with only vanilla Bluetooth 5.)

The 2017 Apple TV has Bluetooth 4.0, so its hardware will not support Bluetooth 5-specific features.