You want them hooked up parallel. Positive post to positive post, negative post to negative post, with the far ends connected as usual to the starter ( + ) and ground ( - ). This provides the usual 12 volts but with double the amp hour rating. This won't allow you to disconnect one battery but that can be simply done with a switch like is used on the back end of drag cars for emergency crews to shut them off. You can mount that switch on your dash and run battery cables to the second battery so you can cut it in and out as needed. The switch is a bit big and ugly for a dash but for an off road vehicle that might not be an issue.
