Govons, A 212 at 4 ohm and a 112 at 8 ohm makes a 2.67 ohms load on the amp. This is good as all speakers, each of the 12's, will get the same power from the amp.
If your amp is 4 ohm max then you can't run these 2 cabs. If you like the cabs I'd get a new amp, the Magellan or Mesa D800 will handle 2.67 ohms, then you can run either or both cabs. A good and flexible modular set up. If you like the amp then you need to replace the 212 with an 8 ohm cab.
The suggestion to change a speaker, means you'll have to change the 212 because it will have to become 8 ohm in order to match the 112. That's expensive, might sound bad anyway and will totally change your sound.
One last suggestion. Buy another amp like the one you already have. Run each amp into each cab and daisy chain the amps.