The box mould is very little work, and gives a lot more options than folding, which I would not recommend. Most important advantage of the box mould is the ability to include landings for the bulkheads and any fore and any stringers in the mould. A 400mm wide hull is hard work to put any of this in, if it is more than an arm's length deep. Using the same bow for both hulls may work as both your hulls are low, but the box moulds are so easy to build that they are probably easier than cutting and shutting.
Rick, thanks for the drag numbers, fascinating as always.