Iam using single Apple ID for two iPhones, but i don’t want to sync the data between them

too many ioS devices this christmas ! My wife's iphone and iPad uses my appleID for apps and purchases but she has her own iCloud acct for pictures, and separate contacts, calendar, imessages, emails, etc.

I need to backup everything of hers and replace some devices. My choices seem to be : make a new appleID for her devices and then use family sharing for sharing pics and purchases (but this kills every app on her phone and iPad... basically have to start over).

The other option is .. can I use iTunes to separately back up two phones with the same AppleID on one computer without merging calendars and contacts and apps ?

I don't think this will work... on windows can I create a separate w10 login, and separate iTunes installation then connect her phone to that iTunes ? just for backup ? I'm afraid that iTunes will still try to merge the phone contents .

Maybe the best plan would be to use iCloud to backup everything on her devices and use iTunes for my backups ? When we set this stuff up, there was no such thing as Family Sharing,... which is how we got into this mess.

Sorry, no. Calendars and Contacts will be shared between the two phones if you use the same Apple ID on both. You can separate messages and calls on either device and manage apps on each device as well. But Calendars and Contacts will be merged.

We share an Apple ID and it works for us to share contacts and calendars.

Create 2 separate ID's.