The smart way to handle this so you don't upset clients who think you are their 'only one' is to not advertise that you are willing to do 2 girl shows. Just do them for the clients that request them. That way you don't tick off those clients, but you still get to please the other ones.
Just my .02
Rick2

To me the real question is instead if you ever play with the girlfriend without a client.  If the answer is yes, then the difference in dynamics will be clear.  If not, then there will be no difference in the service you provide.  Most straight girls are comfortable with some additional female contact, but its not the same as when the ladies are bi.

Consider for a second reversing the genders: you see two guys back to back; how would either session be different if you'd seen them both at the same time?  The guys may also both have seen another lady right after seeing you - how would it have been different if they had seen you both together?

My personal preference: its all about the chemistry of the connection, be it singles, doubles, or groups.  But the enhanced physical dynamics of a group coupling can simply not be denied.

 -- Deep'ly likely to get flamed' Heat