Guy above has the right idea - you can let the PO know you want back to back. Variety is the spice of life.
I will say that some of my best experiences have been with c-girls I’ve had genuine chemistry with. If you find someone like that, my advice is to book the hour. You’ll be able to get 2 shots in probably because at that point, the girl doesn’t want to do the bullshit stuff at the beginning (cat bath, body rubbing, etc). The downside to this approach is it’s a lot of trial and error (so more $).