I've had five, three of those with the same couple.
Only one was shite, and that was the one where we had a girl neither I nor my reg had fucked. She didn't fit our vibe, got pished on vodka and pissed us both off.
The other two ffm were brill. In these, both girls were bi and girlfriends.
With one ffm, I'd fucked one girl already and we got on. Romanian girl in her 20s. In the second meet she suggested her girlfriend join us. I'm glad I agreed.
The other ffm was with a reg who then suggested her (non-escort) girlfriend join us. It was wild. Stopped as it became too routine.
However the sight of them clamped in 69 made me feel a bit left out, even though it was on my request, and so it became all about them on me, and me on them very quickly.
Highlights were:
getting a bj from my reg's girlfriend, as my reg fucked her with a strap on;
bjs and dfk from each girl in quick sucession to make me cum;
casually oiling my reg's body as she made small talk to her gf as she dressed to leave.
So my tip, for what it's worth, is make sure the girls know each other, and that you've met at least one before and got on.