The BSS follows a standard of Master-Slaves concept explained in the Parallax BASIC Stamp Book, as does the SEED and the US40 machines. Pin zero one wire serial half duplex interface bus - absolutely yes. A bunch of Propellers, and BASIC Stamps, connected together - absolutely yes. Distinct and separate boards only stacked - absolutely not. (Even if it was, that would be ok - refer to the discussion on art and visual representation.)
The boards are all connected, software driven at the same time, and talking to each other. Each machine has a specific purpose. Reviewing, for example, SEED software will show multiple Stamps listening at the same time and responding (talking) and processing data simultaneously in true parallel. Programs run at the same time as machines demonstrate various concepts of multi-processing, self enumerating, true parallelism, self evolving, memorizing, AI, determinism, aggregate memory, and so on.