18 hp mercury ignition issues

Have a18 hp xd outboard . It is only running on one cylinder . Which it runs remarkably good on one cylinder minus the lack of power lol . So I put a new coil on it and check for spark and had it with out a load on it .. Took it for a test run and still had no power pulled it back out of the water and checked for spark on the one cylinder again and now it had none .. Switch the power wire from the one coil was sparking to the one that wasn't and it had spark then. Would this be a switch box problem ( which I would call the ignition box but when you look at the part name to order it on here they call it a switch box) I'm leaning this way since once put under load that the one coil Lost spark again but work when switch the wires from the good on it work ... I'm leaning towards that and had somebody tell me that's prolly it but just wanted to double check and see if anybody else had a suggests . The ground , spark plug, plug wire , coil is all good or new.

Sounds like that's the problem. I see used ones cheap on eBay.

Jeff