A fuse is probably not needed because its only there to protect the wire. However you will prolly want to run an isolator to make sure you dont end up draining the primary battery... Careful though, running battery in parallel with different amps may end up shortening the life of both of them...I'm no pro at multiple batteries though
If the batteries are right next to eachother (as is suggested, given the 0.5' distance) and they are properly secured, no, you don't need to fuse it. There should be nothing in the area to damage the wire. Just make sure the wire is secured in the terminals correctly.
Isolators are only needed to separate the starting battery from the audio system when the stereo is being played for long periods of time with the car off.