I would get rather buggypod as weight limit is 20kg and you can always remove it.
It's more comfortable than buggy board.
http://www.buggypod.com/index.php
As far as I know in USA you can only buy it here:
http://www.buggypod.com/retailers.php#USA
There is also Litaf products like Seat 2Go or Pick-up
Last one is buggy board but I'm sure there is more inventions we don't know about so keep browsing and you might find something different perfect for you.
I have Baby jogger city select for my 2.5 year old and 6 months old but obviously the gap is much smaller and I wouldn't buy new double for four year old and newborn.
I'm going to buy buggypod to use for travelling will see if it's any good.
Good luck
Ewa