Sorry for skimming the 2nd amp questions but when I hook up a second amp I prefer to split the power wire with a distribution block.

Basically take the amp main power wire and drop it down to the next lower gauge (higher AWG number) to feed each amp. Put a fuse near the battery for the main wire and one on each amp feed if you like.

Ground each amp with the same guage of wire you use to feed them power, but to be clear it's a ground wire not the same power wire, duh.

As for RCA signal....some amps have a pass through, else you could split one of your RCA feeds like others have said if your HU doesn't have 3 sets of pre-outs for your needs, etc.

And you need to be sure that you have enough power wire to feed both amps. If the amp you have installed already is using the smallest gauge feed wire, ex 8-10AWG type wire, you will likely need a much larger gauge power to split and feed both amps, a 4AWG is like the min size wire for this job.

This also brings to mind that you may need a capacitor in your system as well with the second amp.....

Sorry for butting in.