4 Evenflo Triumphs on sale....or 4 Graco Headwise/Size4Me/MySize on sale... they'd have the same exact 4 seats that will last them each till they are at least 4 years old, probably longer. At least one model of each of those will be available, including tax, for less than $150 somewhere.
The Graco Size4Me & it's clones last the longest of anything rear facing and will last a good long time forward facing, are easy to install, have cupholders, and are narrow AND don't take up a lot of room front to back.
http://www.albeebaby.com/graco-size4me-65-convertible-car-seat-go-green.html
For less money, but really long lasting also, the Evenflo SureRide for $99 (and under, if you can get the version called the Titan Deluxe...make sure it's the same seat first, there's another one called Titan) has the tallest top slots on the market besides a Britax Frontier, so it will take them all the way to a cheap backless booster in 6 years.
With these seats, you wouldn't have to make another purchase in 2-3 years and your twins could stay rear facing till at least 2, probably longer, as recommended.
And of course, you can always pick two that have nicer features & cost a little more for the vehicle you use for longer trips and/or has less safety features of it's own, etc, and the two buudget friendlier seats for the other car.
I definitely think that you should have 4 seats, 2 in each car, even if you go the Cosco Scenera route. It's much safer when seats are installed properly and if you have to move them a lot, it's more likely that mistakes will be made. (-;
(And btw, past the infant seats, there aren't any seats that install with a base...well maybe one...but it's not recommended much and it's expensive...AND you'll like this part, most convertible carseats take up LESS room front to back than the infant seats, so you'll be fine with most any seat you choose.)