Are the 2 - 12's in the cabinet you have now 8 ohms or other?

Usually Sunn amps will take 2 - 8 ohms loads with one plugged into the 8 ohm jack and the other plugged into the 4 ohm jack. The jacks are wired in parallel internally so in essence you are running 2 - 8 ohm cabinets in parallel...4 ohms.

You really cannot assume that 2 jacks mean parallel or series. The only thing to do is check them. You can do this by opening the cabinets and looking or by using a multi meter. Most 8 ohm cabinets will be somewhere around 6 ohms give or take an ohm. Most 4’s will be around 3, same as above. Many cabinets have 2 jacks run in parallel so you can connect the amp to one, then the next cabinet to the first one.

I would think that if your second cabinet is 8 ohms you should be fine. As long as the Beta sees at least 4 ohms, all should be happy. Take the time to review the comments about the Beta controls since the way they work is not like any other amp.