You'll have to cut the SIM from your Bionic down to fit in the MAXX. The RAZR/MAXX takes a microSIM, while the Bionic takes a standard sized SIM. You'll need something like this
MICRO SIM CARD CUTTER in order to put the SIM from the Bionic into the MAXX. Then, to put the SIM back in the Bionic, you'll need an adapter (like the ones included with the cutter in the link).
Once you have the SIM card properly cut, you'll be able to put it in the MAXX. Then, with the adapter, you'll be able to put it back into the Bionic. As long as the SIM itself is active (and it will remain active until you activate a different SIM or a 3G-only device on your account), you won't need to activate it again. Once you put it in one of the phones, it will do everything it needs to do in order to authenticate itself on the network and activate the phone on your account.