Traditional Xinerama distributes 2D commands to a basic MultiHead configuration. Providing the equivelent 3D distribution for the direct rendering 3D stream is new functionality.
This diagram from TG gives an overview of the 2D and 3D streams. The 2D stream is distributed in the fourth box from the top, labeled "Xinerama's Single Logical Screen". The missing functionality, is the third box from the top, which distributes the 3D stream. It is labeled "Chromium-like 3D Command MUX".
Chromium has addressed many of the technical challenges for distributing a single 3D stream over multiple rendering pipelines. However, Chromium is focused on providing this solution over multiple systems in a cluster rather than a single system with multiple heads.