Do I need another battery?

I thought it would be easier to have a second battery because I could just run one power wire through the firewall to the second battery and from their I could run a 4g wire to my amp and a 2g wire to my power inverter. The power inverter I want to run ( Xantrex's XPower 1000) requires its own 2g power cable. With the 3 Xboxes I will be using at least 450 to 500 watts and you never know what else I might want to  hook up latter. Now if I run one battery I would have to run two wires through my firewall all the way to the back. If I do run two batteries what size gauge wire should I run from the main battery to the second battery and how would I go about wiring it? I want to run two batteries and have one in the back for closer wiring but I am not sure how to hook it up so it can still get charged.

Thanks for everyone’s input I appreciate it. I know how to hook the two batteries together if I want, but first I think I will hook everything up then if I think I need another battery after that I will put another one in. Does anyone know how may watts a Xbox uses I thought it was about 200 or 300 just want to make sure and where I could get a good HO alternator for a 99 dodge Durango. I found a couple that put out 170 amps but that is as high is the highest I found. Do you think a 170 amps will be enough? I want to run  kickers KX1200.1, and  A good multi-channel amp but I haven’t picked one out yet but I would like it to put around 50-75*4 and a strong enough power inverter that can  run 3 Xboxes. Just let me know what you guys think thanks and recommend.