The reason for the split is to allow space for the wood to expand and contract. I built a kitchen counter out of old bowling lane one time .Like really old stuff, and I figured it was well sealed and seasoned. Anyhow, I had screwed it from the bottom and my feet told me one day that I had broken screw heads on my floor. I called bologna on the expansion theory until I marked the underside of the countertop and rechecked it on humid days versus dry days. Biiiggg difference there, like almost 1/2". I m not saying that your pine would buckle like a used car salesman will when he has a mechanic for a customer, but I believe that I would experiment with clamping them together for a couple of weeks and seeing what they tell you.
