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If you know which wires do what, that question will answer itself. If you don't know which wires do what, it's time to have a pro installer do it for you. You can let out the expensive blue smoke if you start hooking up wires willy nilly.

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Essentially, both units need constant power, switched power, and ground.

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Then you need to choose which of the two units is going to be responsible for making the amp-remote line go high. That's the one whose amp-remote connected to the amp.

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Regarding the RCA cable inputs and outputs, that's covered here and here.

If you plug on unit into the unit the others Aux in, then the unit "closest" to the amp should control the amp, as it needs to be on anyway to get the sound from the other unit to the amp. (I can't see a scenario where you'd want it in reverse, but I might just be too noarrowminded )

The source switchers solve this as they use the remotes to decide which units signals to pass through, and provide a single remote out.

Finally one could wire them together in an OR fashion with 2 diodes. Each HU remote to a separate anode, cathodes tied together and to the amp(s) remote in, making the amp come on along with either, or both, HUs.