I appreciate everyone's feedback!
Just to give you an idea of our ages, I'm 30, my husband is 29, my boyfriend is 26, and husband's girlfriend is 24. My husband and his girlfriend have been together about a year and 4 months. She just started dating someone else (last couple of months) and it hasn't been the smoothest transition. She is learning how to balance two partners and her communication level leaves a lot to be desired. She doesn't identify as poly, thinks of dating my husband as sort of an exception to the rule. In the future she wants someone all to herself to marry, own a home, and have babies with. Yet it was her idea to rent a house together. Hmmm... even just writing this out, it doesn't sound like the best idea to me either.
I think you may be right. As much as we want to do this to save money and have event space, with all the shifting dynamics, we may need to wait a while and reassess in a few months.