Connect two twin mattresses by using a bed frame that fits the two mattresses and a bed doubling kit. Two extra-long, twin-size mattresses make a standard king-size bed.
Assemble the bed by placing two twin-size box springs on a king-size bed frame, or use only the mattresses for a platform bed. Fill the space between the mattresses by using a gap filler and cover from a bed doubling kit, or cut egg crate foam to fit the gap and tape it in place. Strap the mattresses together with a lashing strap from a kit to keep them from slipping apart. It is possible to use any two mattresses to create a larger bed, but a nonstandard-size bed needs a custom frame and bedding.