If you are talking about only placing one nitrous plate under one carb when you have two carbs? Don;t do it.
If you are talking about using one single plate system with one bottle and only one solenoid for nitrous and one solenoid for fuel but plumbed in to two plates? Go for it. I do it and I like it.
Only problem is the total nitrous power adding capability will be cut in akmost half. So if you have a 300 hp system and install it on two plates with solenoids for one system, you will end up with near 75 hp or a little higher on each plate for a total of 150 hp or in other words a halving of the sum total.
Ed