How would one set up a double battery set up? I have a single battery connected to a solar panel and i can't run my cabin lights for more than a couple hours before it goes out. Or do I look for a more powerful single battery?
You might get a reply on your question if you provide more information. Single battery connected to a solar panel doesn't tell us much. Is it a AAA or an 8-D? Is your panel actually recharging the battery or is it too small and/or not functioning.
You might start with a voltmeter for a rough guess of the health of your battery or better yet check the specific gravity of the battery acid. I don't know how many lights you are using but you could run a few cabin light for more than a couple hours with a pair of D cell flashlight batteries.
Something is definetly wrong with this picture. There could be other electrical problems that are draining your battery that have nothing to do with the lights you are using.
You have a bit more homework to do on this project and marine electrical systems are complicated enough that you may be better served with a good book than a simple answer on this forum. Also do a search of the archives of this forum on the subject and you will find a wealth of information.
Good Luck.