The more toxic E.trigona isn't a cacti, would suggest you should use a cacti base to graft a cacti top. Would think a closer related type of cacti would be more ideal for watering and lighting matching.

Seems to be a novelty idea or cutting corners short cut to it's natural growing. over time any of these types of cacti can develop a trunk on it's own and can be potted as a bonsai with reasonable care

I think Hylocereus (dragon fruit) is a common root-stock.

Josh

Thank you both!

from what I understand about grafting(which is very little), you do NOT want them to callous-- you want to attach them right away. and i guess maybe stretchy garden tape plus stakes could hold them together? but I'm speaking from a very non-knowledgeable POV here! there's a ton of websites dedicated to grafting how-tos out there, they'll def be more helpful than me.

just curious, why would you want to graft? for propagation? or aesthetics?

I've never grafted anything, so I'd like to learn how to do it. And I like anything unusual. I suspect that a beautiful, full rare variety of schlumbergera in bloom would look stunning atop a tall graft. I hope to find out soon! LOL

Slits on the root stock and grafting tape to hold the graft in place for hardeneing may help you on your endevor

Non grafted Rhipsalidopsis