A unified mind can always simulate separate agents.

To within its available resources, sure. But that's under the assumption that there's a categorical difference between multiple agents instantiated on separate hardware and multiple agents instantiated on a single piece of hardware.

Separate agents cannot simulate a unified mind.

Actually, that's the entire notion behind the Society of the Mind: there's no such thing as a "unified mind". Only separate agents that operate as a wholistic system.

If the separate agents all have simultaneous access to the same information that the unified mind would, then they cease being separate agents. In my book, there is no longer a distinction.

I believe there is a significantly false assumption here: that the agents present in human minds are operating with "simultaneous" (or otherwise) access to "the same information".

Furthermore -- the entire concept of stigmergy rests upon the notion that each of these independent agents would produce effects that alter the behaviors of the other independent agents, thus creating an operationally unified whole.

There's a big difference between separate agents all running in one brain (e.g., possibly humans) and separate agents in separate brains (ants).

I submit to you that the differences are more of distance (proximity of computational units) and magnitude ( overall level of currently manifested intellect), and far less of category. That is; I see nothing about the behaviors of siphonophores and ants -- as eusocial collectives -- which prevents in principle a eusocial collective from manifesting tool-making level intellect and/or sentience.