With the empegs RCA out plugged into the other HUs AUX in, the other HUs RCA outs need to go the amps. To listen to the empeg you must also turn on the other HU and set it to use AUX in as source.

With a switcher you avoid the need for the other unit to be on. And it may enhance quality if the circuitry in the other HU isn't as good as the empeg.
You run the RCAs from both the HU and the empeg to it, and then connect the outputs of teh switcher to the amp. Turn on the HU - HU is connected through teh switcher; turn on the empeg instead and it gets connected through the switcher.

The empeg has 4 RCA outputs: front left and right and rear left and right.

If you want to drive more speakers you can either split say the rear outputs with Y-connectors. One branch would go to an amp/channels powering rear speakers and the other branch to for instance a sub amp.
Or the front outputs could be split to go to different channels powering door speakers and the dash speakers...

Two or more speakers can also be combined (in series or parallell) on a single amp output - the problem there is that you can't easily tweak the power level between the speakers.

/Michael