I used the Chicco seats with the key fit. We also used the BOB stroller. I know they make a twin version, not sure if they have the same attachments as the single. I had to buy a snap in bracket for the car seat to click into the stroller. It worked very well.

For the next level, I'm looking at the foonf. It's a sleek 'manly' looking seat

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2005 x5 4.4i sport