I have already addressed whether it is better to have a 'long bike' or a trailer to move a pair of kids, though I am now against the idea of a trailer. I have also looked at tandems, though that is a larger expense for what I'd want. It's very hard to choose between child trailers, 'long bikes' and tandems, because as a pair of kids age the ease of schlepping them decreases, and the amount they want to be independent increases: up to six, they have to be schlepped; from six to ten they want to pedal, but you'll want to keep them on a tandem. I have one child soon 2 1/2, and another soon to be born. Another issue is me (6'+) sharing a bike with my wife (5'1"), though more likely I won't be able to make one thing work for us both.
A Kona Ute set up with a baby-seat for the younger, and stoker-bar for the older one, will cost about $1150, which beats a Surly Big Dummy by a cool grand. Comes in two sizes, but won't be able to share with the wife.
An Xtracycle Cargo Joe folds smaller, which is a neat trick, will get the same use, and comes in three sizes. (One size does not fit all: why I am not interested in the Madsen). It will also cost $200 more than the Kona Ute, set up as I need it.
Then there is this: the Zigo Leader.
It will get only three years' use to the 'long bike's' six, after which I will likely want to get... a 'long bike'. Hmm... Though the Zigo may have 50% resale, as would the 'long bike', not sure it is worth going for. If I'd had twins...