Lately a couple of newer athletes have asked whether it's feasible to compete in more than one Ironman in a year -- Ironman Lake Placid in July and then Florida in November, or perhaps CDA/Arizona. It's tough to know how to answer this question. I've done it twice, IMUSA/Cozumel in 2009, and then Wales/Cozumel in 2011, and I have distinctly mixed feelings on it.
As an initial matter, I firmly believe that most athletes these days jump too quickly to Ironman from short distance races. It's almost like people view Ironman as the only race worth doing, or at least as the clear "ASAP" goal, even if the goal is only to finish under 17 hours. I don't understand why. If the goal is to do your best at Ironman, as opposed to merely finish it, the optimal approach almost certainly is to spend at least a couple of years training very hard and competing at shorter distances first. It's much easier to add endurance to speed than to add speed to endurance. Almost every top-level Ironman triathlete was first a world class short-course triathlete, and the same holds for marathoners, who often were tearing up the 5,000m or 10,000m distances before going longer. People who move quickly to competing in Ironmans often find that they don't get much faster in the course of training for those events.
In sum, in my experience, there's nothing more likely to kill your enjoyment of endurance sports than to train for an Ironman for which you're not enthusiastic, and that enthusiasm very difficult to predict a year or more out. If you do make it through the second Ironman successfully, it's very likely that you'll be extremely burned-out afterward. The most natural thing will be to take a few months off, only to find the next spring that all of the benefit from the work you put in the year before has disappeared. It can be extremely frustrating.
In all, I'd suggest making enjoyment of the sport the highest priority. Life's too short to do anything else, and certainly you should be careful not to get swept up in others' enthusiasm. Make the choice for the right reasons, and be honest with yourself about why you're doing it, what you're sacrificing to make it happen, and what you're trying to get out of it.