There are two systems:-
a) older system: coil on one cylinder firing larger spark plug linked to smaller plug on another cylinder; was to clean up emissions.
b) newer system: coil fires both plugs on one cylinder simultaneously (but coil pack still on the larger plug).
You'll be able to see which system your ph 1 uses by just looking at the HT lead arrangement. Coil packs are not cylinder specific so if you get one suitable for your engine it will work on any cylinder.
On our Bolkow Monsun, like most avgas aircraft, there is a twin spark; two plugs per cylinder and two magnetos. One check is to see if both magnetos are working by switching them off in turn (actually they are grounded), there is a slight drop in rpm when this is done (each mag fires its own set of plugs; one to a cylinder) so obviously the engine runs better with both mags operational, but if one fails the engine does not stop! You're going to be fine running for a little while as it is but I'd get another coil pack soon. I bought a spare new one on ebay for not a lot.