Hi,
I also have the 580 EXII and the 430 EXII and you can actually set which group you want the flash to be in, on the flash itself. I think your dilemma is setting the flash ration between the two with the 580 EXII on camera.
I use a STE2, which controls both the 580 EXII and the 430 EXII and allows me to set the ratio from the STE2 transmitter, but in your case I think you would set your slave (430) full power and then vary the power output of your on-camera 580 EXII to act as a fill light, maybe setting it at -1.5 to -2 exposure compensation.
I hope this helps.
Two Jack Studio