Just run a wire from the first amp's turn on connection to the second one's. Two amps are fine. I would only add a relay with 3 or more. Please make sure your remote turn-on wire coming from the head unit has a fuse on it. I would hate for you to screw up your head unit by shorting out that wire.

If your head unit only has one set of RCA outputs on it, you can use a "Y" connector to split them. This might make your first amp seem quieter though. Be sure to carefully set the gains on both amplifiers to prevent distortion.

If you need more help, read on at http://www.elitecaraudio.com/forums and http://www.teamrocs.com/technical