OS X has the same issue as a lot of other VMs. You can not simply have a single pool of workspaces and chose to view any N of them, where N is the number of monitors that you have. In OS X you can have workspaces on your left monitor, and workspaces on your right monitor, but you can't easily share the workspaces between the monitors without re-arranging them.

The only WM I have used that gets virtual desktops right for multi monitors is xmonad. Maybe dwm can work the same way, not sure.

In xmonad you can have 10 workspaces and 2 monitors, and pull up any workspace on any monitor. You can have hotkeys to cycle workspaces on either monitor independently and a key to swap the two monitors.