IphoneX

It reads like they're asking how two faces can open using Face ID. Meaning they can open it with their face and someone else can also open it.

Did you happen to give the other person your passcode? If they entered it, it's possible that it wasn't even Face ID unlocking your iPhone.

If you're still worried about it, use these steps to setup Face ID again: Get help with Face ID on your iPhone X - Apple Support

You can also change your passcode: Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

That should technically not be possible. Unless you look very much alike and are fooling the face recognition.

Your friend might want to take the iPhone in to Apple for diagnosis. Go with him, so you can show the Genius Bar technician.

After months of having two iPX in our home, neither of them will open by FaceID by the other member of our house. And we even both have each other's passcode.

If this is really happening, there is something wrong with your friends phone and it should be taken to Apple and you should both demonstrate this to Apple.

No. Just one 🙂

It's not possible. The iPhone X can only store information for one face.

If you need a second person to be able to unlock the iPhone, give them the passcode, and they can use it instead of Face ID.

Then your friend needs to turn off FaceID and re-enter his face again. If this happens again, he should schedule an appointment at his Apple Store Genius Bar and show them.

Use the contact support at the top right of this page.