Its pretty common.  You may be able to get a lot more advice on the Provider board but I am guessing you may also get a lot of PM's about this.  I am assuming you mean that you wish to double with another provider with hers or your male client.

*raises hand*

PM me with details if you're interested, or just regular email me

I can definitely think of some friends who would be interested

In both threesomes and foursomes... I take PMs or emails and a get to know you date over coffee before taking it behind closed doors...

At first I thought you meant threesome, which is easy enough to find, but foursome?  Wow.  That sounds hot.