I would say it's either a coil pack gone faulty or you're not getting the switched earth from the ECU to one side of the coil pack.

Check this by disconnecting the plug and using an LED type circuit tester, put the clip end of the tester to the positive on the battery and the probe to either the outer terminals of the plug. You should have the LED flashing. If there's no flash on one of them, you either have a ECU or wiring issue.

If they're flashing as they should, I would say it's a coil issue.