"since B already has a seed" - Confirm you have set the seed on your own, please.

"I cannot just recover sats from A into B" - if wallet B is still empty (zero sats) you can factory reset the device and restore wallet A seed words in it. 


"However, if I never set up B, I could setup and just recover using the A seed phrase on a nee device, but then any other sats would need to be sent to B" - in that case, device A & device B are showing the same wallet. Both devices will receive the sats, sent to one of the addresses. It works similar to your email. You can send/read/write emails on your phone or laptop, using the same email address.

The main question is: why are you changing the devices? If it's because you don't trust the wallet A, restoring the wallet in device B doesn't make sense. It's always much more secure to always generate new keys (seed words).  

 Last sentence (and email example) nailed what I was looking for. Thank you!!