Metro-X supports up to four screens which can all be controlled simultaneously with a single keyboard and mouse. This allows you to run many applications without overlapping windows, and is particularly useful for image-processing work.
In a multiple screen configuration there is one X process, one mouse, and one keyboard - but several screens. The mouse/cursor can be moved between the different screens. Clients connect to the different screens with the syntax -display :0.X where X is the screen number.