This might help:
I get four hits when I search the thread for "Elantra." Nothing specific to two buckets and a Safety 1st seat, but you might be able to get some ideas. (Which one do you have Complete Air or Onside Air or one of the other Air Protect seats? Is it RF or FF?).
I'm sure with the right seats you can make it work. Even if you have to buy the most expensive seats you can find it's still cheaper than a new car.
I would go down to Babies R Us and try a Keyfit in your car. It usually fits in pretty tight spaces, and it fits small babies well which is good when you're expecting twins. Put your Safety 1st seat in the center of the Elantra and try to get it as centered as possible for a good test. See if you can fit the Keyfit on the side (you might try it on each side to be sure). If it doesn't fit, you might have to replace the Safety 1st seat with something like a Diono Radian.
Another infant seat to consider which is just a touch wider than the Keyfit but still a relatively compact seat is the Safety 1st Onboard Air. You might even be able to find it in a pattern to match your current Air Protect seat. :D