Follow the manufacturer's instructions to assemble the bed frame in the spot where you want to place the bed. You can push two twin-size bed frames together, if you're making a king-size bed for only a few nights. But the two smaller bed frames will not provide a foundation sturdy enough for long-term use. A king-size bed frame will help prevent the two twin mattresses from separating and sagging in the middle.
Place the box spring on the assembled bed frame. A king-size box spring will provide the sturdiest foundation for the two twin mattresses, especially if you're using two twin-size bed frames. Two twin-size box springs on a king-size bed frame will be just as sturdy.
Place the two single beds on top of the box spring.
Lay a king-size foam or down-filled mattress topper on top of the two mattresses, if you're only going to be using the bed for a few days. If the king bed is going to be a permanent addition, secure the beds with a commercial bed connector. (See Resources.) Bed connector kits come with special straps to secure the two mattresses together as well as a center strip to fill the gap between the mattresses.
Cover the bed with a king-size mattress cover, sheets and comforter.