I’m hardly the most experienced person on this forum, Ruby, but I’ve done that shot.
You should put the key light far enough away from both, so that one interviewee isn’t over-exposed relative to the other. Put the darker-complected person closer to the key light.
The Rifa, with its built-in diffusion, might be a nice key. But is it bright enough to be backed up a bit and still be the key? If not, you may want to augment it with the other lights bounced into a white card or with diffusion (a clear shower curtain, as long as it doesn’t catch fire, would work). You definitely should experiment before the shoot.
Good luck!
— BC