No, it's not. You need to buy one specifically for the double. It attaches in a different place. I ordered one from Amazon, hated it and only used it once. It makes the stroller really bulky when you try to fold it up. We used a Snuzzler and fully reclined the seat before DD could sit up.
I sent the City Mini back and got the City Select but it's IS confusing about the accessories. I used a snuzzler also and just put DS in the stroller laid back. DS was born in Feb. I just didn't do a car seat adapter. It was a slight pain to not be able to pop the car seat vs the kid into the stroller but only for such a brief amount of time that that is what I'll do again when this next kiddo arrives.
Re: City mini double question
JHB 1/19/09
Baby girl due 11/22/12!