I second the Valco Twin Trimode. Your 2.5 yo may not be ready to ride on a board full time so it's nice to have the option for the toddler seat, and he/she will probably like being up high to see everything--but you could get a hitchhiker too to place on the back. Then when your 2.5 yo is older and basically walking/riding on hitchhiker, using scooter etc. full time you can just remove the toddler seat and you have a good twin stroller for your twins.
Of course when you have the toddler seat in place you won't be able to use a carseat adapter for either twin. I know it's a bit of a PITA but I would just take them out of the seat and put them in the stroller. It's a limited period of time they are in the carseats, as you said. The recline on the Valco is flat enough for a newborn.
I do know the new 2011 Bumblerides take 2 infant seats, but I have no idea how they are with a hitchhiker board.
Also, I would not really consider either of these something you take in and out of the car frequently. For that maybe get a BJCMD and make older kid walk or wear one of the twins, or even a double snap n go (for now, I'm sure you can find a used one) with older kid walking.
M, Mom to 3 handsome boys, '06, '08, '11