Its always better to have a bigger earth wire than the power wire. Just go out and buy your self 5 metres of the stuff and chuck what you dont use in your shed for when you might next need some wire.
I had to do the same as you, I had 2 rca outs on my headunit but had 4 on my amp, just got some high quality gold plated rca splitters. Nothing more required.
If your going to run a power wire from your battery terminal to your amp, don't forget to mount a fuse inline, saved me a couple of times.