how insane would adding 2 hobby rockets to a trainer be?
Just don't do it. Like phayd said, it would require lots of careful setup. Even if they do fire at the same time AND have equal amounts of thrust for the EXACT same time (which is about a one in a trillion chance), you need to make sure they are aligned correctly, which alone requires many years of training to determine the correct thrust/angle/etc... (I think the call them engineers)
Having said that, I think it would be pretty cool. Maybe on a scale C-130 or something. Just wouldn't look right on a trainer.