Here's G. mangostana growing in the same container right next to G. hombriana.
The first picture was taken April 4, the second one was taken today. (That gives you some idea of how much they grow, or don't grow, in two months time)
I plan to approach graft them later.
G. mangostana, as a younger seedling, is really hard to grow. All the conditions are optimized, it's inside an enclosure with high humidity, temperature controlled with a thermostat at 77-79°, and LED grow lights (combination of both red/blue/white), and not too much light either I might add, as I seemed to run into that problem with G. mangostana earlier.
Obviously G. hombriana is the plant that's doing better in the picture.