My other iPhone is stuck on iPhone unavailable screen

I understand that your iPhone is in the state of "Unavailable". You will have to reset the passcode to your iPhone by following the instructions in this guide published by Apple 👉 Reset the Passcode of your iPhone (Without a Computer).  Note: All the data on your iPhone will be erased.

Hopefully, you have a backup created on 👉 iCloud and/or on a computer. If you do then, you can 👉 Restore your iPhone from the backup file once your passcode has been reset and the iPhone has been returned to factory settings by the procedure in the link above.

Axel F.