it is possible and you probably wouldnt have any problems with stacking them but like gimmemud said, you will get better distribution if you just have one plate. Many people run a single plate with two stages of nitrous, just T right off the plate and run two nitrous solenoids and two fuel solenoids each jetted for your stage. just be sure to get a plate that can handle your final HP amount, there are a few 500hp plates out there from NOS, NX, Nitrous Supply, etc.
also, when you install it be sure to have the nitrous bar on top and the fuel bar on the bottom... the nitrous bar will have directional holes that point down and the fuel bar will have holes that point straight out the sides. you dont want to have nitrous coming out of your carb. :wink: