Post this message on a different forum, like http://www.the12volt.com and see how the replies will vary. They will vary because of the background of the people who post. If you want to do this right, the most important thing for you to do right now, is get the proper information. I have seen some good advice above and I have seen some bad advice above. Be careful with the advice you get, research it and find out about the background of the person who is posting it before you take it. It is all too easy for a keyboard commando to ramble on about something that they just read 20 seconds earlier in another thread.
i don't know why people always want to add 2nd batteries before doing other upgrades. if your alt can barely keep 1 battery charged, what makes you think it can keep 2 batteries charged?