if your after MFF then no problem.
about 30% of the girls will do it (for extra money)
your gf has to be confident and look at the girls, make eye contact and smile, then both walk towards the door. tell them exactly what you want agree a price and have fun.
as for escort service email norman
http://www.pretty-escort.com
he has about 10 bi girls 180 euro hour but worth every penny