I don't have mi mix (too expensive for me haha), but I will try answering you based on my experience flashing roms..
Stable ROMs are just older Dev ROMs, after some testing periode by US (Yes, we are beta testers haha), Xiaomi releases it as Stable ROMs, so does Stable ROMs has no bugs? keep dreaming, there will always bugs, just more or less..
Split screen only existed on Android 7 ROMs, for example Mi 5S plus with Stable 9.1.2.0 got split screen.
Afaik, Face unlocking currently only existed in Dev ROM 7.11.16.
A little info based on my experience, you can dirty flash newer ROMs on older or the same ROMs without problem, I just dirty-flashed xiaom.eu 8.5.2.0 on Global Stable 8.5.2.0 (Redmi 3), just wipe cache/dalvik, no problem so far for the last 3 days.
So maybe you can use this info and try the ROMs yourself since obviously there is no much Mi Mix 2 users here...
Btw, what is your current ROM? maybe it's best to provide as many info as you can..