The Inverness Terrace flat that Irina used to work in is an example of not presuming quality congregates. The Thai agencies multi-occupancy flats also support that.
If you get a chance to interact with the girls you're not there to see then perhaps you can make a judgement OR if the girl who's recommending has proven regular material.
If you're seeing a very reputable forum darling then those introductions have worked for others here.
Only reliable situation for upgrading to a duo on site is if mid-punt the flatmate comes in to see what all the noise is about or the girl you're enjoying tells her flatmate(s) to come join in.