We lived in a hippie town in Washington state when I had my first, and the online mom's group I was part of actually had a whole section dedicated to tandem nursing--nursing your older toddler while nursing your newborn. So, yes, it's very possible. However...I saw one mom here say if your periods have returned, you can get pregnant, which is not entirely true. Both a good friend and I had to wean--entirely--before getting pregnant. Now, perhaps if we had been more persistent, we would have wound up pregnant, but they tried for over a year before weaning and we tried 8 months. We conceived the very first month of absolutely no breastfeeding (he was at 4x a day when we started trying...at 2 years old...after a couple months of no luck, we cut them in half, then down to one, and then we conceived). Anyway--it's probably worth considering whether you're ready to be done with nursing, but it's also probably worth a shot at trying to get pregnant while nursing, since it does seem to work for more people than it doesn't. Oh--and what the Womanly Art of Breastfeeding says--is that ANY time you're nursing, even once a day, your fertility is decreased. That, obviously, does not mean "you can't get pregnant," which is why they say so often not to use bf'ing as birth control--but they also say that ending night nursing and going at least 8 hours between feedings is a good start to resuming fertility.
Oh--and I had my period starting 6 weeks after delivery; I breastfed him exclusively for 6 months, and we nursed until 29 months.