Duos used to be my favourite type of booking, but as I've got older, I prefer the engagement of one girl.
What I do sometimes do now is book 2 girls in a parlour, with a small overlap between them so I get a bit of a 3-some but can focus on each one individually for a longer time without paying one to watch.
So for example girl 1 for 30 minutes at 1pm, girl 2 joins in for 30 minutes at 1.25pm, so I end up with a reasonably priced 2 girl for nearly an hour.
Roses Retreat in Manchester are pretty good at arranging this sort of thing.