I bought a bunch of double strollers when my daughter was born, because my son was only 18 months old. I LOVED my Combi for about 8 or 9 months, but then the difference in the weights between my older and younger child made pushing a side-by-side more difficult.
I really love my Kolcraft Contours Options Tandem stroller. http://www.kolcraft.com/baby-products/contours-options-ta...
We still use the Kolcraft for trips to the zoo, etc., and our youngest is now almost two. It is soooo versatile, with a universal car-seat carrier as well as two seats that can be arranged in any number of ways, including facing you, facing each other, etc. It has a ton of space for diaper bags, etc., and cup holders for you. Downsides are that it's big and heavy, and you have to take it a bit apart to fold it up. It's pretty big for just shopping trips or something, and I find it heavy to push on walks when we're going up hills, etc.
Another good option is a Sit & Stand. The storage space is tiny, and the seat for the standing child is not comfortable, but your older child can choose to sit or stand, and it's much easier for the kid to get in or out. I know the Graco brand, at least, takes a Graco car seat easily. I think this is a really good choice for a shorter trip like out shopping or something, but I don't like it as much for outings or times when the older child may get tired and actually want to sit. Good luck.