Meter the BG one or more stops over. Make sure that you dont have your subj too close to the BG or you will have ghosting around your subj. I've used bell-type reflectors and softboxes w/o outer diffusion to light a high-key bg. You want to keep your heads fairly close to the bg and aim them so that they are pointed at the opposite edge of your seamless or cyc wall so that you dont end up with hot spots in your bg.
If your shooting b and w film, only one stop will give you a VI on the tonal scale, I would meter it at least a VII or VII 1/2 and print it down if it is too hot in the final print.
It never hurts to double check via a polaroid before you start burnin through film.