I know in theory you shouldn't NEED 2 cables from the rear battery but I've never had any luck running a single cable and I'm not sure why. I have have racing buddies who have fixed "issues" (slow cranking, poor charging, etc.) by running a ground cable from the battery to a starter bolt (also were I run mine).

I'd say try a single cable and a good ground as described but be prepared to add a main ground IF issues crop up. I'm sort of a freak for grounding stuff after finding that electrical issues are most often the result of poor grounds so maybe I'm just a bit hyper on this.

Dan