Again, the problem is that it's often easier to simply have different varieties on different trees, given those trees can be on highly-dwarfing rootstocks, so that they take up a similar space as a 'cocktail tree' with multiple grafts.
Multiple trees also turn out to be a lot easier to manage -- you can prune them all the same size & shape, for starters, rather than having to track which branches belong to which variety and try to coax the slower growing specimens along, while being extra savage with the more vigorous varieties.
This would be a good way to do it, but extra-dwarf rootstocks are delicate: Poor root systems, poor disease resistance, poor wind resistance, will not tolerate dry places or chalk, and are less vigorous than fruit varieties affecting to the graft point. They typically have much shorter lives.
And not all fruits have this kind of rootstocks available.
Sometimes, in small places, a big tree grafted with tree or four varieties is the right thing to do. It depends on the type of fruit, the type of soil and the space available.
[Cocktail tree is a common denomination for this kind of trees, specially used in Citrus. I have one, grafted by me]
Less vigorous root system, but that's how it dwarfs the plant -- not sure if they're less resistant to disease (I expect this is one of things they select for while breeding). Definitely these types (m9 ,m27) and similar need staking the entire life of the plant.
Cocktail tree is something I've only heard in Australia, and only from some small subset of nurseries -- it's also commonly used in Spain then? A cocktail plant for me would normally be something like Mentha nemorosa (also challenging to find in Australia).
A friend had a 3-way citrus, which did breathtakingly poorly for them, for the same reason my two (3-way pear, and a 3-way apple) -- a single dominant variety and two much less vigorous grafts. My two are now at least 25 years old, and still present more maintenance challenges than some decent stakes, soil maintenance & mulching.
Multiple trees also turn out to be a lot easier to manage -- you can prune them all the same size & shape, for starters, rather than having to track which branches belong to which variety and try to coax the slower growing specimens along, while being extra savage with the more vigorous varieties.