Step, if you do a mag check (left switch off, then on before doing same with right) and you get a drop in rpm when each switch is 'off' (grounding its mag) then you have not got a permanently live mag. If the mag was permanently live (un-grounded even with the switch in the 'off' position) then you'd get no mag drop when you selected that switch to 'off.
This is the same whether you have two toggle switches or a 'left, right, both' key switch.