If it is half or significantly less than her doubles rate there's your answer

Different providers have different rates; I really enjoy doubles sessions so my hourly rate is the same.. But usually I'll specify that my rate is ____ and my partner's rate is the same (or whatever).  

You can certainly guess based on the ballpark rates as published, but if there is ambiguity in what's published it is certainly a fair question. I think most of the time, you can ask for clarification without coming across like a haggler.

If a lady doesn't post duo rates but lists duo buddies it's likely because their rates are very different so you'd be paying each lady whatever their rates are.
I tend to work with ladies whose rates are close to mine (no more than 100 under/over).  
If a lady is typically 600/90 mins solo and she posts a duo rate of 1000/90mins that's what her and her duo buddy would split. I doubt she'd jump up to 1000/90mins just for herself in a duo date.

And of course who the duo is will effect the total.  Usually it is the normal rate for each lady or a bit less.  Just ask the potential duo.

Yep, I enjoy the occasional double, it's something kind of new to me, but with the right peeps, it can be a lot of fun!

I just don't post them since I don't have dedicated doubles partners...