You can connect the beds to one big, cozy mattress. To do so, simply move the silver lever to the side of the bed down to fold down the bed and then just fully unfold the back mattress:
Use the extra mattress to fill in the space in the middle:
Be careful
Never sit on halfway slid out bed slats, as it will bend the extension sliders and cause a costly repair! Do not stand or kneel on the bed as the wooden bed slats will break.
Make sure you do not have someone go underneath the bed while the bed is in use to avoid injuries.
Watch this video for a more detailed explanation: