Inflatable Mouse had been having some trouble getting dual monitors to display video correctly using the NVidia monitor pairing method twinview (not sure if he sorted that out). He was seeing fullscreen video split across both screens. I don't have an nvidia card on hand in a dual monitor setup, so I can't confirm on twinview, but I can confirm that I saw the same behavior using xinerama in some Linux desktop environments/window managers.
However, video works correctly in all desktop environments for me in Arch Linux if I use xrandr for screen configuration. Additionally, two monitors worked correctly for me out of the box in some DEs as well.
In XFCE4 and openbox on the other hand I had to use xrandr to get things to work correctly. I had trouble getting arandr (the xrandr GUI frontend) to work, but it's easy enough to write a script that runs on session login that configures xrandr correctly. Here's mine with comments to help adapt it to your situation:
#!/bin/sh
xrandr --newmode "1920x1080_60.00" 193.25 1920 2056 2256 2592 1080 1083 1089 1125 -hsync +vsync #This line is where you set the resolution of your second display and enable or disable vsync
xrandr --addmode HDMI1 1920x1080_60.00 #You'll need to replace HDMI1 with whatever the correct display identifier for your 2nd display is, I got a list of display identifiers by typing "xrandr" into a terminal
xrandr --output HDMI1 --mode 1920x1080_60.00 #Ditto, replace HDMI1 with whatever your second screen is
xrandr --output HDMI1 --left-of LVDS1 #This places the second display to the left of the primary display (in my case a laptop screen). You'll need to find the correct display identifier for your primary display and replace LVDS1 as well
By contrast, in Gnome 3.12 dual monitors worked correctly out of the box (the video showed fullscreen on one screen only); another forum user reported everything working correctly in KDE out of the box.
It's not clear if future versions of MC Linux will work automatically work with other screen stretching methods, or whether you'll get the same results in distros other than Arch, but if you're seeing this dual monitor issue, it seems that xrandr is a potential solution (or you can use gnome, etc.).