I switched phone carriers, but could not get a second phone number on my apple 12 mini, so I had to buy another phone in order to get a new number for the 13 mini. however the 13 mini does have it's own phone number,need help?

hi, I have a 12 mini iphone that I use daily. I wanted to add a second line to my 12mini, but was told that I couldn't do that by the new carrier I switched to (Xfinity Mobile). So I had to buy another iphone, so I got the 13 mini. My problem is this: Yes the 13 mini has it's own separate phone number, but some how when I went to activate it by following the activation steps like placing my other phone (the 12 mini) next to the new 13 mini which then transferred all of my information from my 12 mini to the new 13 mini. Now every time I get a call or text message on my 12 mini, it also shows up on the new 13 mini? When I log into my 12 mini, the same log in shows up on the 13 mini. How do I fix this I need to have my 13 mini separated from the 12 mini. I need help please and thank you.