And when you're checking for spark then BOTH plugs need to be "in-circuit" or you need to simulate them being in-circuit.
Current/Voltage comes out one side of the coil and needs to get back to the other side of the coil to create a spark. How that works when the engine is working correctly is that the current/voltage goes to one sparkplug, jumps the gap, then goes through the cylinder head to the OTHER spark plug, jumps that gap, then back to the coil. If you remove both spark plug wires and try to individually test for spark on each, it WILL FAIL even with a good ignition system. If you leave the one spark plug in the head with the wire connected to it, then you can check the other wire "to ground" and it should work OK if everything is operating correctly.