Off camera technique & bikini C&C

I'm learning about off-camera flash. I've got a D90 and 1 x SB-600 and 1 x SB-800. I've got one tripod to hold one of them up! I've got the diffuser domes which come with the flash, but I don't have any bigger diffusers or soft box etc.

I've done a couple of photoshoots where I've used an off-camera flash maybe 10 feet to my side as the primary light, but I keep screwing up the second, on-camera flash which I guess I should be using to fill in shadows. I think my flash power on the off camera flash is ok but I need to pump up the on-camera flash power level.

Here's some sample pics; I think the bikini girls look too dark. Pls help out with technique. FYI I also screwed up the whole exposure, but these started as raw files and I did a +1.5 (or so) exposure correction in post-processing.