RCA aux inputs should look like the RCA home outputs on the back of the empeg, ie a stub with a hole in the center. They are usually labeled AUX, AUX IN or something like that.

RCA outputs are normally preamp outputs (nothing prevents you from running speaker lever signals through the same style of wire/connector though)

RCA is the connector style, while preamp means the amplitude/power of the signal (voltage). For home audio preamp normally is 1V, in car audio it can vary. If you start with a "larger" signal at the source you don't need to amplify it as much at the end. In the noisy environment (electrically) that a car is, this means a lessened sensitivity to disturbances introduced on the way from the source to the amplifier.

One pair of the empegs outputs (front or rear) should connect to the AUX in connectors of the other HU, the other pair is left unconnected (good idea to cover them with elecricians tape or similar).

HTH,

/Michael