One password for 2 ID accounts

Have you tried changing the password on one of the IDs to match the password of the other ID?

 Apple ID Password Change.   -  

ASC YouTube for How to reset your Apple ID password

 What to do after you change your Apple ID or password - Apple Support  -   I suggest doing this before you change your Apple ID or password.

This is not possible, you cannot merge Apple IDs

I haven't seen anything that says you can't have the same password. All you can do is try.