Years ago (over 20), there was a place in Boston where if you liked a second girl, you would ask the girl you chose to ask the second girl to join in.   All that was necessary in terms of payment was a tip to the second girl.

More recently, a place I visited would have two girls massaging me when I asked and gave the second girl a tip also.  That was a smaller place that was not a typical AMP, but was a lot of fun to visit.

But as has been said, the best thing to do is just to ask.   The worst they can say is no.