Carley and Doug were both dead by the time that th group met Christa and Omid, but I think they were the replacements for them. Carley was a strong female presence, level-headed and passionate. These traits all describe Christa. Doug was a lighthearted fellow, funny, and had ingenuity. Omid was all of these things (he says they made it as far as they had because of Omid's "scrapper" skills, indicating he's got some ingenuity floating around. And it was made clear Doug and Carley liked eachother, and here it shows these two identical personalities wound up together as well.