Actually I was thinking about buying the VHM heads from Jha. They run the RS type plug on both. I figure I would be cheap and run only one as those RS plugs are expensive.

The recommendation from Jha is to use the short plugs on both but they emphasized using the short plug on the lower cylinder as the front wheel can hit the spark plug under heavy load.

I can't see any reason why I can't run two different plugs as long as they are the same heat range. However, I save like $50USD on not using one for the top cylinder.