B.M.
IF it's the Chicco Keyfit 30..that's one of the best infant seats...I'll post a question over on www.windsorpeak.com in their stroller and carseat board and let you know what I find out.
So it doesn't appear that Chicco makes a double that will take an infant seat..their doubles don't...another option would be to get the other highly recommended and similar seat the SafeSeat by Graco...similar weight and height limits (30 pounds) and very user friendly easy to install etc and very safe....Graco makes a ton of doubles...
I had the Combi Tandem Counterpart double with the matching Combi Connection infant seat (my DD was 3 when DS was born)...it's another great infant seat, very lightweight, narrow base, very reliable, easy to use, safe...I love that the Combi double stroller is very lightweight and folds very small compared to some of the Graco's....you might check it out too.
It seems that Chicco is missing out on a marketing opportunity...their infant seat is awesome but many need it to go with a double stroller like you.
B.