Unlinking 2 iPhones both registered to my Apple I.D.

When I was gifted an Iphone XS I gave my Iphone 8 to my wife. Somehow when setting the phone up for her use I managed to give her phone the same I.D. as mine, so now photos, some messages and various other quirks appear on both phones. When I try to change the I.D. on her phone to her Apple I.D. it says the name is already being used by another I.D. If I login to my Apple I.D. account online I can see her phone under Devices, so I can delete the link and set up her phone using her I.D. right? NO. The Catch 22 is that to change the details on her phone I have to login to it, and as the I.D. registered to it is still mine, once I type in my password the phone is immediately registered back to my Devices. Is there any way to 'unlink' these 2 phones (without having to lose any data) and give the iPhone 8 my wife's Apple I.D. and allow us to live separate lives again?