T.D.
My friend used this one with her twins and LOVED it. It's $100 but will hold both of them and you can use it for a while.
http://www.target.com/gp/detail.html/602-###-###-####-###...
You can also see it on the Baby Trend website, looks like it's sold at Walmart and Babies R Us also. The blue one (first one shown) holds up to 50 lbs. total and the black one (second one shown) holds up to 44 lbs. total. (Although, before they weigh that much you will probably have them out of the baby carriers anyway. At that point it's easier WITHOUT the carrier!)
http://www.babytrend.com/products.php?CategoryID=0002.000...