I would say then go ahead and start trying for number 2. It might take you 1 month or even more, who knows? You can get pregnant while nursing if you've already got your period back which I'm guessing you do cause nursing twice a day probably isn't keeping it away. You might find your daughter weans herself during your pregnancy when the milk dries up or changes taste. Like another poster said, unless you have a history of pre term labor it's almost always ok to nurse while pregnant. Contrary to popular belief, it doesn't cause miscarriages.
Just to correct some misinformation from another poster- Your breasts will make colostrum for the baby whether or not you're nursing. Often women dry up or the milk turns salty tasting as the breasts start making colostrum. That's usually why kids wean during pregnancy. Also, tandem nursing is supposed to HELP with jealousy not cause it. I've read that in numerous places and heard it from people who've done it.
So here's my personal experience. =) I got pregnant when my daughter was 22 months old and continued to nurse her for 3 more months. I weaned her because 2 years was more than enough nursing for me and I wanted a break in nursing before the baby came. Plus, my daughter was such an intense nurser, I just knew tandem nursing was not for me. I would go crazy nursing 2 kids. My daughter was also ready for weaning and we did it in a gradual non-traumatic way. So I was really glad with how it all worked out. I'm still nursing her 16 month old brother.
Good luck deciding and either way, I think it's awesome that you have nursed your daughter passed a year! That's such a wonderful thing.