Most girls have TS friends and many will do doubles sessions. My recommendation is twofold:
First, look for ads that say "2 girl sessions available" or something similar. You will find it on a lot of ads. If you do, call them, it means they can bring a second girl.  
Second, talk to a girl that doesn't have an ad that says that and ask if she has a friend she can bring. Many girls do, and will! Understand that it will mean more money. If she says she doesn't, thank her for her time and move onto the next girl.
This process shouldn't be that hard since you live in LA, which is one of the best cities in the US for trannies. There are loads of girls there and I'm sure you can find what you want without too much difficulty.