John,.John,.............In the future when picking out lumber look at the ends of the board. Both pieces are cut slash grain and if they didn't warp now like they did ,they would have warped later after your sign was done. You should be happy that they saved you from the discouragement of splitting and warping in the finished product.
Look at the diagram Stick put up and when you select lumber pick the boards that have the more shorter cross grain to the end instead of the long grain.
I know it is hard these days to do that , but you won't have to worry about warping and ruining all the hard work you put into a project.
Herb