#1
I know that when two speakers are hooked up to the same speaker jack their power handling is combined, but would that also be combined with a speaker cabinet plugged into the second speaker jack? or does each jack have the same output?

I'm not sure how clear I made that, but the reason I'm asking is because I have a Valve King 212 combo and I'm considering adding a 1x12 or 2x12 closed back cabinet, still using the internal speakers, and 1x12's with over 100W power handling aren't that common
#2
Your combo has an external speaker out. This means that the load changes impedance, which changes your power. But yes you can do it.
Marshall amplifiers are the truest purveyors of rock and roll known to man.

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#3
I know, but thats not what I was asking. Do both speaker jacks have an output of 100W or is it split between them?