I have said this before but what is back to back. I booked 8 sps over a period of 2 days left an hour or 2 between each one. I used to like Chinese Chrystal (retired) and Diamond(retired). I used to see one, then drive the 20 min in between and see the other one, one hour each.
A few times if two girls share an apartment I will do half hour with one then half hour with the other one. Just as much fun (almost) as double at half the price.