I always get 2 sog in 30 mins, usually I ask that if I go for the $60 option that they throw in the extra SOG, at worst I negotiate an extra $20. If the session was really good and the attendant agreed to throw in the extra sog for $60, then I will tip another $20.
The first sog is ok, nothing special, but the 2nd sog for me is much more intense.
I would say that 60-70% of the time the attendant will throw in the extra sog for free if you get atleast a $60 option. 25% will require the extra $20, and 5%-10% want more than $20 or they do not offer it at all.