here's the thing: you're not the first to think of this idea. So look around and see how many real world examples of a touring bike for the two different wheel sizes you can find...

bueller?
bueller?

...crickets...

there is a reason these don't widely exist, and it aint that no-one has ever thought of it before now. But if you really want to go this route, find someone (if possible) who has actually made at least a few of them and talk to them about it. I htink Chas roberts in the UK would be a great place to start... they make amazing touring bikes, both 700c and expedition style 26- and they really know their stuff. they could give you a very informed opinion about why this might or might not be a good idea. Also, the U brake idea, if even possible, is a bad one... U brakes, aside from being very tough to find dont work particularly well, and (usually) have crappy mud clearance. youd be best off with disks, really, or two sets of canti-bosses (front of fork, back of fork, chainstay and seatstay) with one pair for each size.

Pay someone enough money and they'll make you whatever you ask for... that doesnt necessarily mean it will be a good design. I would be interested in what you end up with, but if I were spending my hard-earned, I would pick one (26")