I love my combi twin savvy. The graco carseat isn't suppose to go with it, but if you face it forward it does. It doesn't snap, but has been fine! It is so light and folds down so small. It is side by side, but works so well. Both my sisters have it and really like it. We have 8 kids between the 3 of us:) One of the side by side combis also has a combi carseat that snaps in it.
P.S. Just read a lot of people recommending the sit and stand. I would bring your little one to babies r us and see if they like it. Mine did not. She was 28 months when the new baby came, but wanted to sit in the regular stroller part.
She is 5 now and still can sit in the combi. In fact, I've squeezed all three (5, 3, 18 months) in the stroller at times. (Lay the seat down and the two big kids can squeeze in one behind the other, and the baby gets buckled in his own seat.) I just think it's the best for the price. And couldn't imagine having my 21 month old in a sit and stand with the new baby. Let's face it, some times kids throw a fit in public. At those times it is good to have the option of buckling a fighting toddler in and leaving. I don't know if you can contain them very easily in the sit and stands.