I originally bought two of these thinking I could build a space-saving tower of them.  Ha.  After the cats went sailing a couple of times, I realized that was never going to work as it's all just too light.  They work fine separated, and the cats sleep on them every night.  I covered the top with washable faux sheepskin because I was afraid that the velcro on the cover would catch in the cats' fur.  Plus they are more easily washable.