how to back up 1 iphone on imac with 2 users?

How do we back up/sync each distinct IPhone (we have 2) on their own platform, without getting the other users info on our iPhone (ie” contacts or music)

You said 1 phone in your original post. Now it's 2.

There should be no issue. iTunes can recognize unique devices.

Name them uniquely. If they are both iPhone 8, they are the same phone.

how to back up 1 iphone on imac with 2 users?

How do you mean this? A single phone can't have 2 users. A Mac can, but a single phone should be no challenge for iTunes. If you back it up signed in as John, the backup will only be visible while signed in as John.