how to separate two iphones that are sharing apple id?

On one of the phones go to Settings/[Apple ID name]/iCloud and turn off the switches for the content you want to keep. You will be asked if it should be removed; say No.

Next, sign out of the Apple ID (Settings/[Apple ID name] - Sign Out, and create a new Apple ID. Also change the Apple IDs for Messages, FaceTime, Music, App Store to the new Apple ID if they don’t change automatically. You can then turn the switches on to sync to the new iCloud ID/Apple ID.

Hello lord_geoff,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you have questions about separating data that was shared under one Apple ID. While there isn't a way to do this, if the Apple ID uses iCloud, you could archive copies of the data stored in iCloud and download it to a computer to transfer to the new Apple ID: Archive or make copies of the information you store in iCloud

We hope this helps.

Cheers!