if neither HU has an auxilury input, you can also run a signal splitter switch (availible at ratshack among other places) for the RCAs. you will need to hardwire (power and ground) both headunits in no matter what you do. the only problem is that the RCA selector boxes are quite large and ugly, lol.

the other option is to use an FM modulator to introduce the signal from one HU to the other, but that should only be used as a last resort (horrible SQ and gain losses).