I had exacly the same issue last week, Simple to diagnose witch one has gone down.
Just take the induction housing off and eng covers all 10mm bolts, Then disconnect with the
engine runing each fuel injector in turn (not the coil pack connector other wise eng will just cut out).
and reconnect, the mis-fire will either be worse or stay the same when it stays the same thats the
coil pack witch has gone down.
When changed warning light will still be on just disconnect the battery for 2 mins to reset ECU.
(Never had a pair go down either just the one).
Just for piece of mind before buying you can move the coil pack to another cylinder basically moving the misfire
then repeat above to confirm misfire.