Should I run X 2 spark plugs or Metal plug?

I can't answer the technical question, but I would assume the effects would be minuscule. I have several of these old enduros and some have two spark plugs and some have steel close off fittings. My second unused spark plug is usually nothing more than a simple means of plugging up the second hole. But on a few occasions it has actually helped me get the bike started by switching the cap over to that second plug and it fired right up. Putting a brand new plug in that second hole just seems that it would carbon up and become useless. But evidently not! Anxious to hear from others.