A ppossible solution would be to rip the 16 inch board into 4ea 4" strips or any combination so they don't exceed the 6" jointer. Then flatten one face on each board, joint the edges and then glue it back up to be a wide board again.

If stock doesn't fit on a stationary machine jointer, then you might consider hand planing. I know that this ideas sounds intimidating, but there is sometimes a time when hand tools can make quick work of things. Truing up large stock is one of those instances - especially in cases where this is wind or twist in stock. Stationary planers won't fix this without jigs, and most jointers can't handle larger stock.