if your 2x12 box is 8 ohms then you can plug the combo speaker into a 4 ohm jack and the 2x12 box into the other 4 ohm jack and they will both work properly.
Welcome to the board and enjoy the Rectoverb and 2x12 cab!
Do us a favor...don't sell it! There are several of us on here that have bought Rectoverbs only to later regret selling them. We do tend to keep the 2x12 cabs though