Yes, it may be possible to regrow a coppice from the stump.

I was wondering this too, could it be repaired and continue to live?

Repaired, no. Grow on the stump/roots, yes.

It’s not possible except by regrowing the tree from the trunk. Plants have a much different circulatory system that doesn’t have main arteries and veins that can easily be reattached together. Since they use capillary action to move fluid around rather than a pumping heart, the xylem and phloem are microscopic and distributed throughout the trunk instead of branching like human blood vessels. Aligning enough of them back together to keep a large trunk alive is impossible, especially since trees form calluses over injuries to prevent infection.

How come my grandpad had an apple tree attached to the pear tree? Basically a single tree having branches of apples and pears

Grafting works with small branches because enough of the callus can differentiate into xylem and phloem cells before the branch dries out and dies (via use of plant hormones usually).