Personally, I think the only reason for such a thing to exist would be to have a long boar that could fit within the size restrictions for cary-on baggage, so you should use those dimensions as your design basis. (let's take as a given the convenient fiction that the airline security people will come to their senses and realize that, of the 0.000001% of airline travelers who will disrupt a flight with a weapon, only perhaps a third of those are likely to use a skateboard)
As others have mentioned, the physics of boards being what they are, the requirements for foldability are at odds with that of ridability- you'd probably want to look at stiffer plank-like designs, rather than flexy cruisers. I can think of only two successful board products that split (along different axes, and for different reasons): the Pope Bisect surfboard:
http://www.bisect.com/and the
Voile splitboard:
http://reviews.mec.ca/9421-en_ca/5020-103/reviews.htmIt's probably worth looking at how they solved the various engineering problems. (which are of course different from the problems in skateboards.)
