In all my years of being with trans ladies, I don't recall seeing any teaming up with a GG on the usual websites.
It's intriguing.  Eat the GG out while the TG is banging me?  I suck the TG while the GG sucks me?  I don't see myself doing it, but it's interesting to the imagination.
Of course, I've seen plenty of porn videos posted on places like ashemaletube - I usually just bypass, just doesn't interest **me**.  I know that I get rather disappointed with videos having a title like "Two Trans Ladies Fucking A Lucky Guy" and then one of the "Trans Ladies" turns out to be a GG with a strapon.

Any of y'all ever indulged in such a combo?

They've been around at least 20 years. They possibly died out and are coming back again? They used to be a way for the agencies to introduce trans women as a novelty, they would partner them with the less hot genetic girls in their stable. It was a way for guys to crossover and see a trans provider. If the guy got there and didn't like the trans girl as they thought they would, they still had the genetic girl there so it wasn't considered a waste of time.  

It does happen but its rare, when it comes to the advertising.  

Its a fun time and i recommend trying it.  

The times ive done it, ive found my own gg provider and brought her with me to the ts provider. Have had a blast. Just have to find some open minded people.

Guessing it was just a one night/day or couple of days of teaming up and they went their separate ways.