Been through 5 doubles in my 10 year hobby career. Three were so-so, because the providers either weren't into each other or acted "scripted". One of the good ones is where I met one of my ATF's (a UTR out in SoCal). I just recently set up my latest double with her and my other ATF, both of whom had expressed an interest in meeting the other, and OMG, I was on sensory overload with those two! Unfreaking-believable!
So, the moral of ths story is: put together providers whom you know, and know will work well together.