The first girl should be someone who you have booked before and had a great session with. Let her pick the second. This holds true for both SM and FS agencies.
I once tried picking 2 girls who I liked the look of and booking them together and the results were nowhere near as good as the above approach.
Amazing...as above, one approach is ask a girl you have booked previously who she likes to work with.
However, a mystery double, if the girls align is something else!