i've only seen double lense. when you take out the lense is there like two layers divided by thin foam, if not its single lense and most definitly cheaper. so that most likely means it will fog up easier. did you buy it aftermarket?
if you can somehow seal it 100% completely then that is not a bad idea, but if you cant and moisture gets in they are going to fog like a mother fucker. you can actually sometimes warranty lenses if they fog between the lenses
I don't know about the single crowbar lens, but with my uvex racers, i tried both the double lens, and the single, and the single performed much better. Condensation would get in between the double and take forever to go away.