I’ve also seen a single rate for a duo by two providers who really enjoy each other’s company 🙂.  

So, ask.

Agreed with Knots.  Usually its full price for both, on occasion with full price for both you get a little extra time.

I haven't done duos yet (although a doubles oral session is at the top of my bucket list )

But to answer your question,  each providers individual rate would make the most sense