Surely twin mags + twin plugs should ensure that the engine starts like a car, ie instantly.
Like others said, the issue is normally not the sparks. It's getting the right mixture to the cylinders that's tricky.
Sparks can be a problem too though. The magnetos are configured so that the spark happens about 20-25 degrees before Top Dead Center, which means that the flame front, at normal RPMs, reaches the piston shortly after TDC.
However at very low RPMs, such as when the engine is being turned by the starter motor, the flame front will reach the piston well before TDC and will cause the engine to stop, or even backfire. That's why aero engine magnetos are fitted with impulse couplings, which retard the spark by about 20-25 degrees at very low RPMs.
Impulse couplings are normally only fitted to one set of magnetos though, and it's only this set that is activated when you rotate the mag key to the "start" position. But in aircraft that have a separate starter button (not integrated in the Off/L/R/Both key mechanism), there might be a note in your POH that you should only start on the left (or right) magneto, not on both.