Sounds like you're using Xinerama, instead of your window manager's multi-head support. If you disable xinerama, both monitors on the same workspace. (I'm not talking about a cloned view, either.)
If they are truly separate workspaces (by desktop I think you mean workspace), with separate task bars, then Ctrl-Alt- is probably the key binding will scroll between them.
You might be using Compiz or Metacity as the actual window managers. I use Compiz, and find it highly configurable. You might not have the configuration tools installed to change your default WM settings; install them and explore.