Ok by asking can you hit it twice in one run, are you asking can you have two buttons?? or are you asking can you let off the button then hit the button again?
with a regular single stage plate system, you either have a button or a microswitch on your throttle(to engage at wide open throttle). with the system armed and you launch on your run you either hit the button, or the microswitch activates off your throttle on carb. and it is one single stage of constant nitrous the entire run.. so if it is a 125hp kit you have a 125 from the time you hit the button or microswitch till you let off.
with a dual stage system, you have 2 fuel solenoids and 2 nos solenoids. that way you can run either two buttons or a button and a microswitch on the carb throttle linkage. you hit the first button and say it gives you a 75hp hit then a little ways down track you hit the other button and it adds another 125...for a total of 200HP shot. it is mainly used when you want to run a big shot but traction wont allow you to use it all at once.
But they also make progressive timers to slowly ramp up the nitrous flow on a single stage kit. But thats a whole nother book..