Some people have done it, but it will really depend on the specifics of your rig. Do you have the right room, to get the right slope, to move your drain to where you want it? In most cases where I've seen dual sewer outlets, it's because the axles happen to be in the way. If you can get past them (with the correct slope for draining) then by all means, go for it. If anything, you could actually gain a bit of grey water space that way, with the extra volume of the extra run.
Oh, and of course, if you do do it, you have to come back and post some pics.