Wondering how rates are set for these sessions. Is it as simple as paying each provider their normal hourly rate? What if one of the providers normal rates are higher than the other providers? Split the diff? Thanks in advance.

.......ask the whoever you're setting things up with. If they work together often they probably have a four handed rate.  

Whether you call it doubles or four hand or whatever, I am one rate. I don't even work out money with anyone else. My clients have to cover that with the lady. I don't do specials, and I don't up sell. I get my own stack, and that's it. I like to make things simple, and I don't like to spend any of my time talking about it during a session for sure.