My understanding is that you can't have two SIM card's use the same number, so I'd have to call and activate/deactivate each time I wanted to switch phones?
I believe that is no longer the case, at least in Spain.
Certain operators offer a 2nd SIM card (with the same number) for a few extra euros a month.
In that case though, you can only speak through one of the phones at a time; however, both benefit from the data connection.