Hi colinfromwellingborough,
Thanks for posting in Apple Support Communities. It sounds like you're referring to calls and messages in particular when you say that your iPhones are connecting. In that case, it may be your iMessage settings. Do you use the same phone number for any settings on both iPhones? You might try disabling iMessage on one of your iPhones. Otherwise, iMessages will be delivered on both. Check out the following from Add or remove your phone number in Messages or FaceTime - Apple Support.
"Set up your phone number
On your iPhone:
- Go to Settings > Messages and make sure that iMessage is on. You might need to wait a moment for it to activate.
- Tap Send & Receive. If you see "Use your Apple ID for iMessage," tap it and sign in with the same Apple ID that you use on your Mac, iPad, and iPod touch.
- In the "You can receive iMessages to and Reply From" section, make sure that your phone number and Apple ID are selected.
- Go to Settings > FaceTime and make sure that your phone number and Apple ID are selected in the "You can be reached by FaceTime at" section.
On your other devices, you can now turn on the phone number that you just set up:
Take care.