I have done both ways.
We have set a few cars up using one set of solenoids to perform two functions (turbo car.) The last car I did was pretty trick...
-When you pull up to the lights and stage, there are two arming switches that need to be in the "on" position.
-One switch activated the nitrous when it saw WOT, and transbrake input, but it was set on an RPM window switch so it dropped out 200rpm before the chip (we did this to spool a fairly large turbo on a 6 cylinder)
-The second switch would turn the nitrous back on when the transbrake was released, and still saw WOT, bypassing the window switch and running the spray down the big end.
We have been running this setup on the car for two seasons and haven't hurt the trans/ converter yet to speak of. Its a quick and easy way of killing two birds with one stone using the same pair of solenoids!