There's another option apart from running one units outputs into the others AUX in - use a RCA switcher. Essentially 2x4 RCA inputs and 4 outputs. Which set of inputs that get switched through to the outputs is determined by the remote leads from the HU, where one has priority in the case both units are on. Also gives 1 remote wire output (effectively ORed the remotes from the two HUs)

/Michael