You have two of them. If one fails you can continue to fly on the
other. Best thing to do is replace ONE. Then you have one new one and
one old one. I consider mags to be 2000 hour lasting accessories, or
replace when they fail. You could easily argue that you don't have to
replace them until you rebuild the engine. It really is up to you. But
with mags, you do have a backup if one fails and the likelyhood of both
failing at once is small.