Cheapest and easiest is parallel, one connected directly to the other. You want a fuse before the second battery as well as after the first in case there is ever a short in the wire. Running in parallel, you need to make sure both batteries are good or the bad one will pull down both batteries. Adding a second battery can help with voltage drop, as long as you're not exceeding the amperage draw your alt can handle. If you are your batteries will simply drain and not be getting a charge. First be sure you have a good battery up front, and have did the big 3 and have a good ground in the rear for your amp(s). If you then want to add a second battery, go for it.