I just ran into this post over at Mint Arrow. The Baby Jogger City Select is on sale on Amazon! The City Select is THE best inline double stroller. It's usually over $600 for the stroller and second seat- this is a steal!
I have also seen them for sale on Craigslist in good condition. A stroller doesn't have to be brand new- many people take great care of them. Buying used can save a bundle of money for the things you DON'T want used- like a carseat. (Second hand carseats may have been structurally compromised by a car accident. Unless you know the full history of the seat it's recommended that you buy from a retailer. )
Check out all the ways that two seats can be configured: