What if I can't sync two iPhones with the same iTunes at the same time?

When I connect my iPhone 8 to iTunes on my PC with Windows 10, it shows it's connected as normal, but the Sync button is grayed out and disabled. That tells me that I can't sync my iPhone 8 after I have started syncing my iPhone 13 Pro Max with that iTunes. Is there anything I can do to be able to sync music for both iPhones?

I had done an upgrade to my iPhone 13 Pro Max. After the upgrade was done, Apple Music seemed to have transferred from my iPhone 8 to my iPhone 13 Pro Max, but everything in Apple Music on my iPhone 13 Pro Max was grayed out. I had to actually plug it in the iTunes that I used with my iPhone 8 and sync it before the music on my Playlists would show un-grayed.