Switching all open windows on 2 displays with one keystroke
You can set a shortcut to move among your screens. In fact, the monitor you add is just considered as another Desktop space. So you can perform the following steps:


Open System Preferences > Keyboard > Shortcuts tab;
On the left menu, select Mission Control;
There will appear some options. First of all, check that the Mission Control function is enabled. There will be consequently shown some other sub-options, such as "Switch to Desktop X", corresponding a certain shorcut. The default one is ctrl+X, where X is the number of Desktop to switch to. You can customize this shortcut as you prefer.


Then, you can just perform the shortcut corresponding in switching to the Desktop space you want and it's done.

If you don't know/remember the 'identification X' of your screen, just show your Mission Control (in my case pressing F9) and check for the Desktop number.