emac power cord work with a g5 dual 2.0?

I would have thought so. Both machines have internal power supplies so the cable is just that, some cable to get power from the socket to the transformer. Note I have never done this so it's at your own risk!

A power cord, is a power cord, is a power cord. They're all the same, an eMac just uses a normal kettle lead right?

My eMac power cord has some plastic around it so it sits flush into the side of the computer, but I think the prongs are the same. You may not be able to plug it in all the way because of the plastic, though.