Try the message board for the Baby Bargains book at
www.babybargains.com They have a separate board for stroller discussion and probably half of the posts are regarding double strollers (and those people take strollers seriously and have probably gone through 5-8 strollers each in search of the perfect one). There are a number of strollers that are also single strollers to which you can add a toddler seat. you can probably get the pros/cons of the most highly regarded doubles which usually include:
Mountain Buggy Urban Double (this and the Valco are probably the most highly regarded)
Valco Runabout Double (or single with toddler seat)
Peg Perego Aria Twin (they have some tandems too, like the Duette)
Maclaren (I forget what it's called)
Phil & Ted's e3 (my personal fave and pretty cool looking)
BumbleRide Queen B (with a toddler seat)
Zooper Tango
Threaded Mode