Yes they are. If you look in my gallery you will see pictures of my cage when it was 2 levels. I didn't use any connectors, only zip ties. The thing to remember is to use lots of them! Every corner had 6 ties. 4 like this:
and then 2 more diagonally making an X. I did use a broom stick as support poles for the middle of the upper level because it sagged ever so slightly, but I found the boys tended to knock the poles over so they could play with them.
Currently I only have a single level but I am toying with the idea of adding a second level again but haven't decided yet. I will post pictures when/if I do.