There's a possibility that the Velocita won't fit through all standard doors-- as there are a number of reports that it feels a bit wide compared to similar models (e.g. the Inglesina Twin Swift and Maclaren SBS's), but I can't give you a definite answer on that one as there just isn't a huge amount of consumer data on this stroller. From what is out there, it's clear that it's a fine double stroller, although the canopy design is less than perfect, with reports that it can spontaneously pop off after a few months of use (Maclaren has the same problem).
Maclaren's Twin Traveller is designed for infants and 24 inches wide-- which is plenty narrow. I'd recommend the Inglesina Twin Swift, but it really is designed for six months and up.
There aren't a lot of lightweight side-by-side models which can accomdate an infant carrier. Actually, I can't think of a single one. It's very difficult to create a lightweight double stroller that could handle the uneven distribution of weight resulting from an infant carrier. I know the
Mountain Buggy Urban Double has this capacity, but we're headed into heavier more serious all-terrain stroller territory there.
Have you looked into
Phil and Ted's E3? It's a fairly wonderful stroller for growing families, although you might find the jumpseat a bit odd, all the reports from parents are that their toddler just loves to sit in it. And yes, stay away from the tandems-- they're a bear to steer when your kids get older,
In the event you simply can't make up your mind in time, there's always the first 4 to 5 months with a sling and single stroller.
Good luck and let me know if you have any further questions!