I don't understand why there are eight separate Harmony's in such a small space -- all controlling only two devices. If the devices are all very close to each other, can you use the "blaster" function to control the two boxes and skip your proposed emitter junction?

mml, do you already own the Harmony's? What sort of user interface are you proposing? Phone/pad App? One of the Harmony controllers? A combination of controllers?

Full disclosure: I'm not a Harmony person. This is not a case of my being a pro and therefore automatically rejecting anything used by DIY's. Harmony limits my ability to creatively design user interfaces. I will say that the few customers who have installed their own Harmony systems love their system, but from my viewpoint only a mother could love that child.