Well, I don't think that that amp is going to pull anywhere close to 550 amps. But if that's your thinking then yeah you need the proper fusing you don't split the fuse in half or anything. You use the appropriate fuse for the wire size and if you add another wire which is the same size, you need the same size fuse. If you are going to exceed the rating of the wire then you need a thicker wire. You say that you are going to add a second battery in the rear and you want to want to wire to that.
So you run the thicker wire all the way from the front to the back battery then If you are only doing a short run from the battery to the amp, you might be able to get away with a smaller wire for a short run.
I run a 1600 watt monoblock and I run a door speaker amp which is about 400 watts RMS the main fuse that I use is 150 amps then I have a distribution block which is fused in the rear which has a couple smaller fuses. I don't seem to have any problems sure I could put a 250 amp or 300 amp fuse coming from the front of the vehicle but that's unnecessary I do not pull anywhere near that power. I don't think you're pulling more than 300 amps with that setup. The sub amp probably pulling 250. door speaker amp probably pulling 50 at most
