The wood grain is going length wise. I am more familiar with wakeboards and they have zero flex. My kiteboard is a really old Slingshot Manta and that also doesn't have any flex. I would say the flex is similar to a snowboard.
The 2 layers of glass will stiffen up the board a bit, but flex in a board does help landing jumps, carving turns and getting through choppy water.
IMO 2 x 1/4" ply bonded together would take a lot to snap.
Another option for even more strength is to bond the 2 sheets with a layer of glass and epoxy between them first.