I've been standing at the background with a camera remote and then covering the background with a black velour type fabric between first and second exposures.
From my searches this morning, the mask mode probably wouldn't work with the method you and I use to extract subjects. It fires groups sequentially.
Whereas we would need A and B (A=subject, B=background) fired for the first shot and A only for the second.
Do I remember correctly that you are using something other than photoshop?
I use Gimp, but any app with real layer ops can serve, including that Gimp clone called Photoshop. The black-drape trick is very effective—alone or in combination with extinguishing the b/g light. Sequential firing is fine, and exactly the same as MultiMAX SpeedCycle. If the b/g is captured in its own frame, that becomes the alpha channel after suitable curves work. I use either approach.