Put on gloves and safety glasses. Sand the two boards with 180-grit sandpaper until smooth. Wipe with a tack cloth to remove all dust.
Brush a good-quality primer onto the tops and sides of the boards. Allow to dry, then turn the boards over and prime the bottom of the boards.
Paint both sides of the boards with three coats of a black high-gloss latex enamel paint. Sand lightly between coats according to the manufacturer's directions. Set aside to dry.
Turn the bookcases upside down. Mark screw holes for locking casters on the bottom corners of both bookcases. Predrill the screw holes, then screw the casters securely to the bookcases. Turn the bookcases right-side up.
Set the bookcases side by side, butting the sides together tightly. Mark screw holes for three hinges on each bookcase to join them together. Predrill the screw holes, then screw the hinges to the bookcases.
Fold the bookcases together. Apply a bead of construction glue to the tops of the bookcases.
Align the painted black boards on top of the bookcases so they butt tightly together at the center and have equal overhang on every edge. Press firmly onto the construction glue. Open the bookcases and from the inside, predrill a screw hole in each corner, through the top of the bookcase and into the boards. Screw together with the wood screws.
Open the folding bar. Adjust the shelves to fit your bottles and accessories. Fill the bar with glasses, liquor bottles and bar essentials.