One iphone, music purchased with two differnet itunes accounts

I have an iphone 15 and it has music on it that was purchased with two different itunes accounts. About half of the music is from my primary account and the other half is from an older account that I still have access to and still use once in a while.

I remember in the past being able to sign into the music with one account, then signing out and signing back in with the other account and the phone would ask me if I wanted to keep the music on the phone.

Then, by virtue of having signed into the phone with both accounts, all of my music was available. Over time, I have forgotten exactly how i did this so I was hoping someone here could help.

So, in short, I have music on my phone from two different accounts. The music purchased under the current account is there, but the music from the second account is grayed out. Can someone remind me how I log in (is it just in itunes, just in music) and get access to this second cache of music from the other account?

Thank you.