I would invite the two of them to coffee/dinner etc.
I would throw it on the table that I was hearing rumors that there was some lying going on the in their dynamic-I wasn't comfortable with it because I didn't want to hold either of them responsible for something that wasn't true.
Then I would point blank say "So and so told me that Erika you are seeing Ned and (Ted? I can't keep these names straight sorry) you made it clear to me that you believe the two of you are monogamous.
So I felt it was best for ME if I got you two together to clear it up so that I don't get dragged into the middle position of "keeping secrets" for one person from the other or believing bad things about one of you that are incorrect.
Then shut the fuck up and take a drink of your coffee. Let them go from there.
"Love As Thou Wilt"