CAB M Merger : Smart solution for running multiple speakers

I have a Two Rock Classic reverb 1x12 combo and sometimes a want to use a external cab to get some more stage volume. I want to use both the internal speaker (8 ohm) in the combo and the external cab (8 ohm). The problem is that the amp has (like many other amps) a set of speaker outs - a 4 ohm, a 8 ohm and a 16 ohm jack. If I use the internal speaker the 8 ohm jack is used. If I was to insert the external cab (8 ohm) into one of the other jack there would be a miss match. This is where the cab merger comes in. It lets you insert both the internal speaker and the external cab into the cab merger and then you run a speaker cable from the cab merger into the amp giving in my case either 4 ohms in paralell or 16 ohms in series. Great little devise!