If you have a band saw, you can cut the two legs at the same time by screwing them together. Create the top of the seat back using a 2X6. Contour the seat back to create raised backrests for two people. For strong joints, use wood glue and thin wooden wafers known as biscuits. You 'll need a biscuit joiner to cut the slots for inserting biscuits.
Wooden dowels are an acceptable alternative to biscuits.
Lay the supports in place and mark the locations for cutting the biscuit joints . Place the seat support 13 inches from the floor and the side support 5 inches from the floor. Clamp the pieces in place until the glue has dried. Be sure to wipe up any excess glue before it dries. Make sure that the pieces are square and clamp them together.