Purchase a king-size frame and mattress bridge or connector. The connector is so you do not fall through the center of the twin beds. Both can be obtained from a bedding store. Assemble the frame per the manufacturer's instructions and place on the floor.
Place the box spring of each twin XL mattress on the frame. Make adjustments to the frame, if necessary. Place the mattresses on top of the box springs, making sure they are even with the sides of the box springs.
Place the mattress bridge on to connect the mattresses. Depending on what type of bridge you purchased will depend on how to install the bridge. Make any final adjustments to the box springs, mattress and the bridge. Dress the bed with king size bedding. Your twin XL mattresses are now a king size bed.