can two people(face IDs) be setup on one phone?

I'm not sure about that specifically, but you can try to set up an alternate appearance for the other person. Follow these steps:

1. In Settings, tap Face ID & Passcode
2. Enter your passcode when prompted
3. Tap 'Set Up an Alternative Appearance'

I'm pretty sure that 'Alternate Appearance' is not meant to be used that way.

It's more likely meant for beard/no beard type situations.

If teaching an alternate appearance as a second person, I suspect that Face ID becomes less secure.. as you'd be telling the iPhone here is 2 different ways this one person can look.

As the information under 'Set Up an Alternate Appearance' says:

'In addition to continuously learning how you look, Face ID can recognize an alternate appearance'

I don't think alternate appearance means alternate person, or they would say it...

no

No problem! :-)

– Jay (they/he)

I understand that. However, the original poster probably knows that letting someone else unlock their iPhone will come with weakened security.

– Jay (they/he)