Lana, the person to ask is Meg (Hotlips Houlihan). See her thread titled "My female on female cherry was popped last night" fron 6/26/2010).
The link is:
http://www.eccie.net/showthread.php?t=65584
HH is not bi and does not do doubles, but again, like everyone states, you have to feel comfortable and it depends a lot on the circumstances. You don't have to feel compelled to do anything, but you should discuss this with the client first, just so his expectations are not deminished should you decline to put on a show with the third particpant for the benefit of the client.
Me? I haven't ever done a F-M-F event. I think this is every guy's fanatsy, but few of us ever realize it. . .and yes, most guys, to include myself, we want to make sure the girls enjoy each other and meet the guy's needs.