Multiple variety trees

I have been asked to graft a tree, on M-26 rootstock, of 2 different cultivars, (which, mercifully, are of similar vigour). Now I need to know the details of how the folk who provide the big box stores with multi-graft trees do it.

Normally, one grafts one's chosen cultivar onto the rootstock just high enough from the soil line to avoid scion rooting. Seems to me, in the current instance one has two choices: 1) graft one of the chosen cultivars onto the rootstock low, let it grow and form a framework, then top work half the branches with the second cultivar, or 2) select a 2 year old M-26 root, which has already developed a beginning branch structure, and top work the whole thing with the two cultivars. (ie. the trunk, at least up to the first tier of branches is M-26.) (And I do in fact have a 2 year old, branched, M-26 root available). Any experience or advice?

An alternative idea is to bud an M-26 maiden in August, with buds of the two cultivars, at suitable height to form the branch structure, (next year). Again, a trunk of M-26, with branches coming directly off it with the desired cultivars.

These trees are widely sold - how do the commercial growers do it?

Broomholm Orchard
Zone 5b in Nova Scotia

The multi variety trees that I’ve seen look like they’ve been budded, each variety marked with a different paint color to quickly label them in the nursery. I’ve only made a handful of these trees for friends in town that only have room for one tree. I use failed grafts from the previous year, and whip and tongue graft on to three or four branches, looking to create a balanced open center tree or a central leader type with a few different varieties at the bottom and a single variety top.