Sorry should have said, yes its one coil that doesn't fire. If you swop the two signal wires over that go to the DIS pack the opposite coil stops working, and the coil that wasn't firing starts firing. Showing that both coils work, but only one signal wire is sending a signal from the ECU.

Presumably the crank sensor being a single unit (it is the crank sensor that tells the ECU where the engine in, which in turn tells the DIS when it needs to fire?) wouldn't stop the DIS working like this?