or save the hassle of running a 2nd wire back to the deck. just connect a 2nd wire to the original amps remote terminal and go from there to the 2nd amp.
You don't want to do this with more than 3 amps though, as the current draw required to turn on multiple components can exceed the supply of the HU's turn on circuit.
If you ever get past 3 amps you could save yourself a possible blown remote output by connecting a relay.