The only purpose of fuses is because there is no way to turn a battery off. Once a wire touches ground, if there is no fuse there your car is history. So, you NEVER need two fuses on the same line. Just keep it as close to the battery as possible, because any short between the battery and fuse obviously will not pop the fuse.
You can rid yourself of several fuses there if the rear batts are spaced out. I don't see why your rear batteries are that spaced out. You should only have 1 fuse on a power/ground pair. So when you connect your front to rear batts just fuse the positive. Then, keep your rear batts as close as possible(touching each other physically) then just use a short wire connecting them. You CAN fuse this wire although if something is shorting between the batts, chances are it is shorting the batts themselves too and you're screwed. Then you need a fuse after the rear batts and that's it. You really only need 2 fuses.