jabarr's comments are right on the money, technically. Your mag toggle switches need to apply ground to the P-leads to stop the sparks from being made. Do a search on the forums here for ignition toggle switch - a few minutes of reading will save you hours of re-wiring frustration.
More importantly, grounding out the P-leads is something that must be done with surety. Failure to ground a P-lead will result in a "hot" mag, and, ultimately, somebody will get hurt or killed by touching a prop when they thought the ignitions were turned off. Having two switches to control the mag doubles the probability of a switch failing to make contact, resulting in a "hot" mag. Not good, plain and simple.
I don't know if you've ever tried holding two switches while jockeying a throttle trying to get a start. This is a recipe for disaster - sooner or later you're going to let one switch go, and either stop cranking or turn on the right mag during cranking, resulting in increased kickback potential.
Put your ignition toggle switches in an easily accessed place. Turn the left mag on, hit the starter button. Once the engine is running in a stable fashion you can then take your hand off the throttle and turn on your right mag.
Don't think this will work? Take a look at all the Grumman Travelers, Cheetahs and Tigers. That's how they are wired. A 4-position key switch (Off, Right, Left, Both) is used, and beside it is a momentary contact starter button. Starting procedure is to turn the key to the Left position, hit the starter button, and once the engine is running, turn the key to the "Both" position. Those airplanes have been flying like that since the early 70's and nobody complains about not getting their engines started. The engine will start and run fine on the left mag for the few seconds it takes to get the engine stabilized and to move your hand to the toggle switch. I say this from personal experience, starting a Grumman in some of the more difficult starting conditions (OAT = -20 degrees C). This process works. You don't have to bring the Right mag on line the instant the first spark plug fires on the left mag. Take your time, flip on the right mag once the engine is running. It will become quite automatic. If you're worried about running on only one mag because you forgot to turn on the Right toggle switch, well, you ARE going to do a run-up, aren't you? You ARE going to cycle both mags off and on, so you'll catch this little lapse in attention, should it ever happen.
The momentary mag switch on the throttle is, plain and simple, an unwise idea. At its worst it's not just unwise but is an unsafe idea. The control of something as critical as ignition is where you want to apply the KISS principle to the max.
