Slide the two twin bed frames next to each other. Line them up so that both ends are even and touching each other.
Measure the distance between the sideboards on the bed since they are rarely lined up with the end posts. Measure the length of the gap between the beds.
Use the measurements to choose a board depth to fill the space. For instance, if you have a 1/2-inch space between the sideboards, choose a 1/2-inch thick board. Use the length of the gap to mark the length of the board, and cut it to the appropriate size with a wood saw.
Slide the board into the gap between the beds. Slide an adjustable hand-screw clamp under the beds so the clamp faces upward and holds the two sideboards together with the gap-filler board in between. Hand tighten the clamp only enough to hold the wood together.
Add two or three more clamps to the sideboard, spacing them evenly from each other. Hand tighten each using the wooden handles until you have securely clamped the bed frames together.