That should work... just make sure to line up the center grow rings, put the cuts together quickly without any drying, make sure the cuts are clean, and make sure it's held securely for a week or two. Use something soft and stretchy, like old stockings or a soft ribbon.

A potential problem might be that the Fairy cactus has so many branches......it may be more likely to put its energy into its own branches than the red Gymno graft. I don't know if that is the case or not, but a lot of times you can not have side branches with the stock/base plant or the graft will suffer. I know a lot of people graft onto Myrtillocactus, which are common and relatively fast growing but still more appropriate than the Hylocereus which are generally used for mass commercial production. Still it would not hurt to try it on the Fairy Castle and report back here how things work out!

Okay awesome, my main reason for this is the plants slowly dying. If I take off the healthy small balls off the red one and cut a smaller stock part off of the princess castle would that work? I figure I'd have to cut the fairy castle stock then root it and then cut the top and put the small balls onto it at that point? Or would this even work since they are both so small?