well i cant think straight right now, but if i get what your saying.... you only need one output, you can split the RCAs and run to each amp or some amps even have an output so you can run the RCAs to one amp and then form that amp to the other amp.
i would think that they would have different signals because rca's send the music to the amp to send to the speakers and different speakers get different speakers im pretty sure they would be different
the only difference would be between the left and the right signals. front and back should be the same. cd's for example only have stereo sound. sub output maybe crossedover or boosted by the head unit though.