Probably wouldn't be all that hard to sidedress with the two tanks on 60" gear, but it would be pretty hard to keep them upright. It's actually pretty hard to keep a single tank upright on some high clearance 60" gear. I don't really see any reason you could not build a double tank setup with one tank behind the other and concentrate on keeping the CG fairly low so that it could operate on 60" centers as well as a single tank will. There are lots of older sidedress tanks in Nebraska which are mounted on 3 wheel gear, with a 11L 15ish front tire and two 13.6-38ish rear tires straddling the tank. The tank itself is mounted low to the ground.

Actually there is someone in the general area, (like i've heard people talking about how stupid he was nearby, but not in my immediate area where I can actually speak to having seen it nearby) who built double tanks on a 120" gear to sidedress with. The "he's such an idiot" part comes from the fact that the dealer's scale apparently isn't wide enough to weigh them. I have no idea how they weigh a floater or sprayer on a narrow scale that is not above ground level, apparently they haven't thought of that yet.

I'm trying to find pictures of the Nebraska tanks but I don't seem to know where they are stored....searching the hard drive now but they may be on CD somewhere.