If planting closely, prune them like a hedge so they merge with each other, or if planting in tree-like groups, prune each individual tree so that each one grows into its own space (ie, Pluot from 1-3, peach from 4-6, nectarine from 7-9, cherry from 10-12) without encroaching on the airspace of the adjacent neighbor tree.
If planted on ththe same rootstocks, then I think they don’t compete with each other or wncourage each other. I’ve heard that different rootstocks help growth through competition although I’ve read on this forum where people have tried multiple rootstocks and something fails completely which is I assume an example of a weak rootstock being killed off or outcompeted by a stronger rootstock.
All hearsay. I haven’t tried it myself