Can someone confirm for me an assumption?

After some tinkering with mate references in assemblies I have come to the conclusion that they do not work in parallel braches of the assembly tree.

For example I have two sub-assemblies in a blank main assembly (template with some mate refs to the planes pre-existing) that always go in the same place when building a new machine tool (front plane/top plane and right plane/top plane) and then everything else is mated to these two sub-assemblies (machine axes, base etc.). I haven't figured out how to get the first assembly to snap to mate references (always goes to fixed either where I click or at the origin) but I have figured out how to get the second one to auto-snap to the selected planes. After this I sort of expected to insert references to features across subs in parallel and have them snap to the subs already in the main (i.e. snap machine axis with bottom/side/bolt hole ref. to same refs. on headstock).

I was under the assumption that we could treat our subassemblies like Legos and have them snap to each other when they are pulled into existence in the same main assembly with the properly named/matching mate refs. in each sub.

I’m guessing this can’t be done?