Brian wrote:

It is not possible to have a single tree with other than all apple, all
pear etc. A single tree can, though, have different varieties of the same
fruit. It was probably this that you read.

Why not, out of interest? IIRC scions of different fruits are grafted
onto the same rootstock, so that shouldn't be the problem. Is there an
incompatability between scions of different fruits?

James