Although the Hylo is a epiphyte, its also still a cactus. Treat it like a cactus that needs a little more water.
The Gymno is a naturally occuring mutant that does not photosynthesize on its own. The Hylo is supporting both plants. When the Hylo fails, which yours is, the Gymno will die also, unless regrafted. All plants grow from the tips. The Hylo's growing tip has been replaced by the Gymno. The Gymno continues to grow but the Hylo can't. So the same amount of Hylo is supporting a bigger and bigger Gymno.
Three years is pretty good for this plant.
Daisy