This is not two plants grafted together, this is Melocactus zehntnerii the lower green part that looks like a barrel cactus is its juvenile body, the upper part is its adult body, the apical cephalium.
 

Discocactus hortii the white flowers flower buds will form from the cephaliu.

Cephalium is a woolly and bristly swelling at the top or side of certain cactus species in which flower buds and fruits are formed, it is frequently brightly coloured.. Melocactus have this densly spined crown and can take a number of colours, forms and shapes. The cephalium will only begin growing after a cactus has reached maturity, at which time the plant quits growing. No other family of plants has members that undergo such a striking phase change
The areoles at the crown of the plant modify to form a large amount of wool and bristles. This cephalium can grow for many years. Once flowering begins the flower buds will form from the cephalium , and because it is located at the upper end of the shoot, it is an apical cephalium.
The largest part cacti undergo little or no noticeable change in growth pattern once they undergo their phase change from juvenile to adult condition: an old, adult plant capable of flowering looks just like a young, juvenile plant. However, in cephalium-bearing genera like Melocactus, the phase change is accompanied by a dramatic change in growth pattern and body form, The adult body form differs so greatly from the juvenile that many people assume that the plant is actually two separate genera grafted together. Excellent examples occur in Melocactus. All species of Melocactus grow initially as globose cacti with spherical, green bodies with strong, widely spaced curved spines that protect the shoot but do not shade it significantly. The plants grow like this for years without ever flowering. Once old enough , the plants switch to being adults the apical shoot starts to produce the cephalium, it became narrower than that of the juvenile, spines being shorter and straighter and so abundant that – combined with the close packing of the wooly areoles completely protects the adult surface from animals, sunlight, fungi, bacteria With no light whatsoever penetrating the mass of spines and the cephalium completely lacks chlorophyll.
The protection offered by the cephalum is remarkable and also its morphology makes less expensive for the plant to produce a flower.
But the positive benefits of having an apical cephalium are accompanied by negative consequences as well. Once each plant switches from the juvenile to the adult phase, it becomes incapable of producing any new chlorophyllous, photosynthetic tissue.

Apical cephalia occur also on Discocactus and Backebergia. Discocacti are small, globose cacti like melocacti, but plants of Backebergia are giant, highly branched columnar cacti.

(See also pseudocephalium)