This is a hard one, I'm interested in the same thing. Master-peace (Jumal) has two red flags who sleep with other people, Sex Drop (Shikke) has some promiscuity and yellow flag behavior from both sides, Jealousy (Scarlet Beriko) has two red flags who both sleep with other people... To a degree The Pawn's Revenge (Evy) may fit the bill.
Most of these kind of have much more intense plots than just the characters being players, but I guess that's what you get with two red flags.

Any BLs where they both are players?
Or both lie? Something both as red/yellow flags