J.R.
Just wanted to say YEA! that there are other tandem nursing moms out here! It has been a good experience for us too. I just tried to keep my older nursling to her 2 times a day (morning and nap), though it didn't always work out that way. And I had a problem with OVER supply -- with all the nursing between the two of them, I was producing tons of milk.
If you do decide to wean, just make sure to hold her more and give her more attention in other ways. She will adjust to a new sibling soon!