Nice cabs the Mesa 2x12 (one sealed one open), not a small box though as as you say got some weight...
Hey El, I had a Yamaha quad box (now owned by MikeB) which I wired stereo and put some cardboard down the middle (I split it left and right, not top bottom), it worked fine/great although not quite as good as 2 ADA split stacks. I've also got another single split stack which I've wired stereo (less to carry...), but nothing inside to separate the speakers. I've found it flanges a bit when I set chorus depth 100 rate 0 (a trick I always used with MP1 to get a fattening delay), so I've recently turned off the chorus to get less flange (comb filtering) interaction in the single box.
A pair of 2x12 Mesa cabs (as above) would sound killer... If you go for open back, the speaker generally needs a stiffer cone as it's got no air suspension (not to mention they spill front and back).