Sharpend
I would advise only using the left mag, part of the job that the impulse unit does is retard the spark to just after TDC thus all the effort of the first power pulse is to send the piston in the correct direction.
The right mag will always fire at (depending on the O360 variant ) 20 or 25 degrees BTDC and try to send the piston in the reverse direction if it fires at the slow cranking speed. So at best having the right mag on during start will do nothing at best and at worst it will appose the efforts of the starter motor.