THey have a new double stroller at Babies R Us that does just that. It's made by Kolcraft. It's really cool, they can face eachother, sit stadium or you can even take one of the seat out and use it as a single stroller. There is an animated link at Babies R Us that shows you it's features. Mind you, I did not read the reviews, but it looks great! http://www.toysrus.com/product/index.jsp?productId=2646209
However, if you want something much more lightweight and manageable you may want to look at the Joovy Caboose. It's a sit and stand stroller for the older child (does have a belt to secure them). It's much smaller than a double stroller and my son loves to ride on the back. My oldest was just shy of 2 1/2 when my second was born and it's been great! A friend has the Baby Trend sit n stand which is what they sell at One Step Ahead and it's great too but much, much bigger and bulkier.