I dunno, and don't feel I'm in a position to comment directly on a SD class specifically involving two or more persons working with firearms.
FWIW
Seems to me that you see in both hunting, SD, and target shooting maybe less so, husband-wife, Father-dau. Father-son, there is noticible participation. You probably have some sense of how common or not that is from the places and classes you've particpated in or run.
Teamwork and interaction with other good guys/gals in SD situation or even a potential SD situation is really important.
Two or more people working together is a huge force multiplier. That alone might be enough to resolve the problem in some situations.
Such thinking runs against the perceptions and beliefs of big swaths of the American public, and maybe more so amongst gun owners. We've become a very individualistic focused society. No need to go into the possible reasons. It's just a reality that might play into the interest (or lack of) and experience in working together. Those with military experience and some team sports probably being the major exceptions.