you NEED to have fusing, or breakers... Because you have a battery at each end of the cable, you need a fuse or a breaker ("protection" from here on) at each end. If you do not, and a short occurs in the middle of the cable, having protection at only one end still leaves the other battery connected to the short, and it will continue to short out. Having protection at both ends, if the cable shorts in the middle, both fuses will blow or breakers will open, and the batteries are now protected from the short.
Protection should be within 18" of the battery at each end. I say 18" because within that distance, it is very unlikely there can be a short between the protection and the battery. My personal preference is a cooper bussmann breaker, it should be at least 175 amp if you ever intend to use the aux battery to "jump start" the main battery.