Help with achieving this effect

[Buckster]

Might give you some ideas. Though for a shot like this you might find it a lot easier (and cheaper if you don't have all the lighting gear) to use layermasks in photoshop

Plus, he's shooting against a black background with all the ambient light in the room shut off, so that it's just his flashes at work. I'm not seeing how this method could be incorporated in the original example.

Enjoyed the vid though! I love watching McNally work!

 

[Overread]

True it might be very hard to do outside - the only way I could imagine doing it would be to use snoots on the flash lighting so that it was lighting the subject only. Then combine that with an ND filter so that the exposure for the ambient light stretches out to the point where its into a few minutes. At that point ambient shouldn't be strong enough to light the people and they can move into the frame - be lit by the flash and then move out to let the exposure finish.

That said it might not be practical - areas around their feet would still catch flash light and muddy the exposure there, so you'd probably still be back to 2 exposures so that the area around their feet (ie ground which would get ambient and flash) could be corrected.