You can use a router to joint two boards at once. This is a really handy trick if you don’t have a long-bed jointer or you don’t have a jointer at all, and you’re working with really long stock.
The trick is to rout both boards at the same time, so the edges mirror each other. Bowed or not, they’ll always fit tightly. To set this trick up, mill three spacer blocks to 11/16 in. thick. Clamp the boards to a pair of sawhorses with the spacers between them.
Chuck a 3/4-in. bit in your router. Clamp the guide board so the bit takes an equal amount off both boards, about 1/32 in. Ride the router tightly against the guideboard, removing the spacers as needed. Any deviation or wandering from the router will be mirrored on the opposite board, providing a perfect fit.
– This trick first appeared in American Woodworker.