Trike ???
 

I'm curious, how effective would it be to find a cheap trike, take off the fenders and put some 28 in tires with knobs on? Also, anyway to convert the drivetrain provides power to both tires? Might work if on snow and trails where the tires can catch up in turns, but I'm guessing that's just playing with a huge mechanical failure if you tried combining them. Maybe 2 sprockets in front driving 2 chains to 2 seperate hubs? Just thinking and killing time at work. Thoughts?

Try http://www.longstaffcycles.co.uk/ for info on differential drive for trikes

Not really an answer to your question, but trikes can go in snow.

There is a Catrike forum where you might be able to get some answers.