Absolutely, positively, do not fly on one mag if the engine is designed for
two! The second mag is not just for redundancy.

The two spark plugs light off the cylinder charge in two separate places.
Ever heard of burning your candle at both ends? The combustion takes place
faster so the burning is over when the exhaust valve opens and things have
time to cool down a bit. With one mag, the exhaust will be hotter and, in
some cases, combustion still taking place when the exhaust valve opens.
This can burn valves. On some engines, it can even damage the exhaust
piping.

Flying even 40 miles this way could be burning the engine's candle at both
ends and maybe even your own.

--
Roger Long