Add me to the list of people who were familiar with polyfidelity, but not polyexclusivity or polyfaithfulness.
You could also just say you are closed, or exclusive. No need to add "poly" in front of it if you have already established the fact that you are polyamorous. Lots of people say they're in a "closed triad" for instance, in your case it would be a closed Vee instead.
If we're talking about labels, "metamours" would be the shorter word for "they both have me as a partner but aren't romantically or sexually involved with one another".
This being said, when coming out, you have to explain everything anyways, so the words themselves are less important, what matters is just explaining the situation. Lots of people understand situations better than labels anyways, especially if they have wrong assumptions about what the label means, so I find it's best to just explain things rather than try to briefly describe them with labels the people might not be familiar with.