Two tenons. Literally, "hands." Projecting rods, such as those common in our dinner tables, seem to be meant. They may have been of metal, let into the boards to a certain depth, and projecting several inches beyond them.
Or, possibly, they may have been of acacia wood. In one board—i.e. "In each board"—no doubt, at the bottom of each. Set in order one against the other. Arranged, i.e; at regular intervals, the position of each corresponding to the position of its fellow.