Did you boil the sugar in 2 cups of water? If you did that, then you'll be fine. Despite what a lot of noobs think, the boiled priming solution is of a similar density as the beer, when you are flowing the beer into the bucket, it easily mixes and mingles the two fluids just fine.
If you're talking dry sugar, it's never a good idea just to dump it in......it won't dissolve well regardless.