mike, it looks like a pretty strong flash was used.
in this case, an immediate automatic darkening of the BG with no consequences could have occurred just by increasing shutter speed. doing so has basically no effect on the flash illumination because the flash itself is so quick, but the "ambient" light will be affected. you will be limited by the sync speed attainable between camera and flash(es). this has the advantage that it is free.
a second option is to close down the aperture. this has the DISadvantage in that the flash light IS affected, so you'll either need to increase ISO, or increase flash power output. the first option introduces noise and decreases DR, the second decreases battery life and increases recycle time. not quit free, but it still doesn't cost any money.
the final option is to get faster glass, and shoot it wide open for shallowest DOF. work close to your subjects with a fast, long lens and everything else "disappears." this costs a lot of money!
you did a decent job of masking, which is 99% of the battle. in fact i did not immediately notice they were masked, and i'm pretty critical! that same mask can be used to burn down the BG after the fact.