A.S.
You might try one of the Sit & Stand strollers. My older daughter was three when my little one was born and we love our Sit & Stand. You can buckle the older one in on the seat. We use ours all of the time. My 4-year-old gets upset when we don't take the Sit & Stand and she has nowhere to ride (I wish I had a place to ride too!).
We got the one that is sold at Target or Walmart (don't remember) as the back seat it larger, but Babies R Us also sells them. Make sure if you go to Babies R Us that you get the one with the adapter for the infant seat. They will fit basically ANY baby carrier.