As long as you can handle it, that is great! Eat super healthy and get plenty of vitamins. I nursed until my son was almost two, and soon after got pregnant, now I am nursing that one and probably will go just as long.
Again, it just depends on your body. If you are healthy and committed to taking care of yourself, you should be fine, and the benefits to both babies would be something to commend you for. Your body will tell you how much is too much, so pay attention, but if you seem to be fine... well, you get the idea by now, I'm sure. Good luck!