I've got the Bertini Jet too, and a friend of mine recently used it for 2 13-month-olds with the toddler seat - she found it worked fine. There's always twin strollers, if you plan on doing lots of walking but not lots of shopping! (that way both kids could be under a sun shade or storm cover if need be - that's the only problem with toddler seats). I suppose you could always buy what you decide on for both kiddies when the time comes, and sell off the Bertini which probably wouldn't have decreased any further in resale value over that time anyway.
Hey correct me if I'm wrong, but isn't it Mountain Buggy that you can buy a 'conversion kit' for to turn it into a 4-wheeler that can then take a toddler seat? Is yours a Pedigree? You can definantly convert them and put a toddler seat (not the board) on.