How do I swap numbers on 2 of my devices.
First, make sure the lines in question are NOT under a contract, because if they are, you can't make any changes whatsoever without breaking the contract.
If the phones are using eSIM, then you'd delete the eSIM on both phones and then do the "activate or switch device" to download a new eSIM for each phone where you pick the other line in question so the phone is then moved to the other number. If the phones are using actual SIM cards, Verizon doesn't support moving SIM cards around anymore and it might "break" the phones if you try swapping the cards because that happened to another forum member some time ago when he tried moving his old SIM card to a newer model of the same brand phone. My recommendation would be to visit a corporate store and get new SIM cards, then do the "activate or switch device" thing in My Verizon if your phones do not support eSIM. Otherwise, remove the SIM cards from the phones, and download a new eSIM for each phone with "activate or switch device" using the other line so the phones are swapped.
If you're prepaid, you have to go to a store or try calling Prepaid CS at 888-294-6804. People have reported elsewhere the My Verizon device swap function doesn't work for prepaid accounts.