DsiplayPort and its variant Mini DisplayPort are interesting protocols because they use a packet interface to send mostly the changed data. There is no gratuitous repainting the screen with the same data (every 1/60 second or similar) as performed by almost every protocol that came before it.
This would free a lot of time on the interface for the information to support an additional display in the newly freed "dead time" when nothing has changed on the primary display. What would be needed at the computer end is a different address for the second and subsequent display(s). What would be needed at the Display-end is a display with a "daisy chain" output port (it cannot be the same as an input port because the signal drivers are set up to accept input, not drive output) to be able to pass the signal along to another display.
To the best of my knowledge, Apple has not yet taken advantage of this ability to address a second display, and at this writing these interfaces on Apple computers support only one Mini DisplayPort display.
There may be the ability to daisy-chain a second Thunderbolt display off the first Thunderbolt display in a chain, when working with a Mac's Thunderbolt connector.
But at this writing, any Mini DisplayPort device connected to that port is always the last device on the chain.