I've never had one myself but I did see a guy getting a two girl massage at an amp one time.  The weird thing was that the door was wide open to the room.  Certainly wasn't going to be a lot of traffic and the guy had a towel on but you'd think a little more privacy would be desired.  I thought it might have been a cultural thing, made me want to get multi cultural real quick.  

I'm sure it's available for the right price but I've never asked.  One of the problems you'll run into is there are often just two girls who work a typical spa and they won't both want to be tied up in case another customer shows up.  You'd probably have better luck getting to be a regular at a spa where they know you by name and then calling ahead to try and schedule something.

I'm wondering if any women on the board have been to an amp.  I talked to an amp girl about this one time and she told me it happens all the time, if the girl comes alone and looks like a lesbian they'll turn her away and make some excuse.  She said it was common for guys to bring in girlfriends and wifes for a massage and they did cater to that.  She said the tips are better and that it mostly involved a body to body rub with a finger finish for the girl while the guy watched.  Sounded like a heck of a way to kill an hour to me but I haven't generated the courage to talk to my SO about it.