I had a similar problem. I bought a 3 1/2" inch thickness and had it split on the bandsaw mill. By the time I got it home there was a one inch gap between the two boards. Mine was wetter on the outside than the inside. I fought those two boards for a good while without success. Finally one night thinking about it I decide to take charge over those boards. The next day I put saw kerfs on three inch centers on the bottom of each board about half way through the thickness. Then added five battens crossway with slotted holes. Then added two more battens that are part of the base support structure. Problem went away. All this was about four years ago and it is still fine.

Ken