Well if you are talking about a monitor as in display then they aren't serial. They are normally VGA, DVI or HDMI. The VGA looks similar to serial at first glance but you will notice it has three rows of pins instead of the two a serial interface has. You can hook up two diplays as long as your OS supports it and you have either a video card with two outputs or two video cards. Winblowz makes it pretty easy if you have the hardware, you pretty much just have to plug the monitor in and tell windows to use it.