What seems like a simple question is not that simple at all.

Crabapples, sometimes peaches, more often apples are available as multiple grafts. In your photo #2, what allows a perception as a lateral arrangement is the branching of the plant as it grew, with each separate scion seeking out it's own space. In multiple apple grafts, this is usually even more apparent as the grafts are spaced and then often trained in espalier fashion.

It's far more complex in the conifers. The conifer picture you show has what looks like chamaecyparis on the bottom, juniper in the middle, and boxwood low. the boxwood is growing on it's own root in the ground, like normal. Then you have two separate stems growing up, one for the juniper and one for the chamaecyparis. I've seen versions with a third, topping out (in the nursery) at over 8'. And yes, it had a third stock for the extra graft.

There's a practical answer and a theoretical answer. the practical answer is, no.

The theoretical answer is if you can find plants close enough to each other, yeah, you could do it, but then you run into issues of vigor and overcrowding.

Ever see a dwarf alberta spruce with a reversion growing out of the top or the side? A dwarf alberta is a white spruce, so a white spruce does quite well sharing the root stock. Not so well sharing the rootstock of other species. Most often how I see this represented is on blue spruce where the graft was a little high, or someone missed a branch in pruning the rootstock, and there's a green bottom to a blue plant.

Ever see a red leaved Norway maple, or a variegated one, where the colored branch is small compared to the green monster it shares a rootstock with? Sure, you could find a blue spruce and green spruce and a golden tipped spruce and put them on the same rootstock, but they are going to grow at different rates, which is going to end up in a one with greater vigor swallowing those with less.