Yep, they changed the rule last season. Single speed is now legal for dual. In fact, I raced my cruiser in one race last season and actually ran over the original poster (dan_wask). Little bastard ended up winning because he got up faster than me. I'm old.
Im gonna go single. But it does get tricky in certain situations. I think if you can rail tight berms at speed you can do well with a single, the only issue would be getting a good snap out of the start. So then you think, why not run a semi easy gear to favor the good start? Which I kind of wanna do, but some of these tracks end up having a pretty good decent where a harder gear is key. F**k it, I,m riding trails!!! or go carts