Two HUs to one amp is probably easiest and cleanest solved by a Sony XA-39II switch. It gives you 2 sets of 4 RCA inputs and 2 inputs for remote turn on. On the output you get one set of 4 RCA outputs and one remote turn on. One of the input sets is "master" - if this remote turn on is active its RCA channels will be switched through to the output independent of what the other input is doing. If inactive it allows the other inputs through.

/Michael