I've had more than a few duos at the saunas over the years including at London Street.
From my experience if you want it "straight" where there's no girl on girl action then you would simply pay each girl the normal hourly rate but if you wanted "lesbian" then it is/was £20 extra per girl..WARNING..some girls' definition of lesbian varies somewhat with some girls like a certain long serving Hungarian at Blair Street simply taking the piss
..while others are suprisingly "full on"..like a certain long serving Scottish girl at Blair Street
You might as well get your moneys worth mate cos even if you just want the other girl to watch she is still going to charge you the going rate.
Oh and another thing..make sure the girls are comfortable with each other or it could be an expensive mistake.