It'll keep you a lot more motivated than running by yourself, and it's a lot more fun. Plus if you're a techie, it's a good way to get outside and meet people.
FWIW, the 2013 Chicago Marathon and the 2013 NYC marathon are pretty close together for a first-time marathoner. I would suggest just doing one or the other.
If you get really into marathons, there's a very interesting subculture of people who do them all the time - for instance, the Marathon Maniacs - http://www.marathonmaniacs.com/