Almost any spa (legit ones too) will be willing to do 4 handers.
It's double the money for them for the table and the hour,
and it makes both the manager and girls happy because more work and money is being spread around.
Sometimes, if it's slow, they won't even charge you for the 2nd house fee so long as the 2nd girl gets a nice tip too.
You just have to establish yourself as a good reliable safe client over the course of a few visits.
Some of my best sessions have been unplanned spur-of-the-moment 4 hand sessions.