Does the 100c have the capability to run a second 2x12 cabinet? It surely has enough power since the head is also 100 watts.
Short answer: no.
The CODE100C / CODE100H do, however, have FX send / receive (post DSP) so I guess you could hook up a second amp. and cab. but that's not what you're looking for I'm guessing i.e. may as well just buy 2 x CODE100C / 2 x CODE100H. OR you could get "creative" I suppose i.e. hook up 2 pairs of 2 x 8 ohm cabs. in series (16 ohms) and hook up these two pairs in parallel (bringing you back to 8 ohms). Probably sound great with four cabs. But again I'm guesssing that's also not the answer you were looking for.
I recently bought the Code 100 2x12 combo and it has a 8 ohm output jack on the bottom right rear of the amp. From what I have read online you can use the Code 212 cabinet or the Code 412 cabinet. If I understood your question correctly. To my knowledge the Code 25 and Code 50 do not have this output jack. Hope this helps.
ps-I freaking love my Code 100 2x12 Combo...sounds great and has that classic Marshall bottom rumble or growl...love it!!!