Pinky
In all probability having the right mag on as well as the left doesn't achieve anymore than just having the left mag on. What I mean is, having the right mag on in most circumstances is pretty harmless as it's very unlikely to produce a spark so the risk of a backfire and kick back is pretty low.
The reason for the lack of spark is the right mag isn't rotating fast enough to produce enough electricity when you prop swing. At the risk of telling you how to suck eggs, the impulse coupling goes two jobs, retard the spark for starting and momentarily spin (impulse) the mag fast enough to produce enough power for a decent spark. This I believe is why it hasn't been a problem for you with both mags on.
However I think there is always a risk, albeit very small, that the right mag could produce a spark so if I were hand swinging I would leave left the right mag off if at all possible, unless I knew it had an impulse coupling as well.