I have an unusual issue between two iphones....

I have a 8plus iPhone I upgraded in July to an iPhone 12. The 8plus service is tracfone and the 12 is xfinity. I transferred the phone no from the 8 to the 12 and had tracfone assign a new number to the 8. I gave the 8 to my granddaughter. I am still logged on to apple on both phones. The phones are sort of linked where everything I do on the 12 shows up on the 8 and visa versa. The 8 will not let me sign out from apple. Is there any way to uncouple the phones so everything doesn't show on each but they will still work the way they should? In the settings on the 12 it shows the 8 as on my account. I removed it from the 12 but the 8 made me sign back in. My granddaughter has an apple account for an iPod but the 8 phone won't let us sign her in to apple. So....is there a way without completely resetting the 8 plus to separate it from my 12 iphone so when my phone rings...hers doesnt?