I’ve been frustrating like you coz I have one Apple ID with two phones two numbers to keep my work life and personal life separate.
I didn’t even realized they merged until recently coz they never did - anyways I was doing some testing while talking to apple support (who didn’t help and just kept repeating the same message) - coz I wanted to see how soon the call logs would merge
Then I stumbled by chance that they don’t sync right away - rather, the two phones would first automatically connect because of the new NameDrop feature of iOS 17 and then once they connected, the lists merged!
now that I have turned off NameDrop, the lists don’t merge anymore!
Settings > General > AirDrop > turn off “Bringing Devices Together”
I hated it anyways coz whenever I put my two phones together it would make that sound and that screen thingy I just never got around to turning it off
And It’s a feature I can live without - I’d rather have my two lists stay separate.
Hope it helps!!