http://www.velorution.biz/?page_id=1190
They have advantages in that the kids are in front of you, and with young children this means that they can see you as well.
The occupants are better protected (not that trailers are a problem) and also with the cover weather protection is good.
Cons are size for storage and cost.
I don't know about two child seats, may be worth inquiring though.
Not a lot cheaper and slightly less bulky are the Cargobikes from
http://www.dutchbike.co.uk/Bikes/cargobike.htm

