Pack it with flowers of sulfur and keep the fissure dry. Other than unsightly, it should be OK.

I don't have sulfur powder. I do have some sulfur pellets. I might be able to grind a few in to a powder but I'm not sure what else might be in the pellets. They're meant for adjusting soil pH. Anyway, the split is calloused a bit already. I think if I don't bother it and keep it dry it will be OK.

I thought that might be the case, having seen other Cintia self-seal after bursting (so maybe it's a genetic thing). It's available at a compounding pharmacist's, and is quite useful for packing plant wounds, especially after surgery.

Update: to me the split didn't appear deep enough to interfere with the top growth point however currently there are two pups forming- one on each side of the split. I'll provide pics tomorrow if I remember. The other cactus in the pot still hasn't split and isn't pupping.

Bifurcation is a blessing.