wow, that is going to be quite a project but it's good to see someone bringing that car back to life. As for getting dual batteries I wouldn't waste money on that right now since it will be awhile until you will be using them again. I would just borrow a batter from another vehicle, no need to have dual since they were just for weight distribution. Just be sure to disconnect the other cables so they don't short out.
I'm not certain that trying to get the engine running will be worth the effort. If it sat so long you should tear it down completely for a total rebuild.
There is a lot of rust on that car. I'm sure you will be a welding expert by the time it's finished
Good luck!