You can get two (maybe more) 'shallow' dives out of twin twelves. I would imagine most people do more than one dive at anyone time so if you're lugging two single tanks along anyway why not bolt them together and give yourself a bit of redundancy?
As has been said, it's worth having some consistency kit-wise - otherwise you're talking about doing potentially your most challenging dives in your least familiar rig.
Just my thoughts...
And howdy - welcome to the board
