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You can do this numerous ways with varying degrees of perfection and strength. First, you have to make sure the boards are flat and don't have a noticeable cup to them. They also have to have a straight edge, which they usually don't unless they have been run on an edger. If they fit together reasonable well (again, this could vary from building a shelf to making a fine cabinet) you could simply apply would glue, line up the edges and clamp them with pipe clamps. If you want reinforcement, dowels or biscuits can be used. Dowels are simplier because all you need is a placement tool to align your drilling, a drill and bit, and the dowels. The biscuit jointer is better, but then again, you have to borrow or buy one. If I need a board wider then 12", I usually use veneered plywood, cut and rip the the size I need, and add a cover trim piece on the edges. The plywood usually takes out the issues with warping, cupping, and uneven seems where they have been joined.