it wont hurt anything stacking them, welding them together isnt a bad idea though, as long as your not spinning your tires, and takein curves at like 80 - 90 mph you will be fine... Oh yeah avoid POT HOLES
I have a 4 and 2 stacked in the back, haven't had any problems out of them...just check to make sure they aren't shifting around on you every now and then and you'll be straight.