Multiple display support seems to be a major feature regression from v11 to v12. The 'Display Options' box is similar, but the way it works is very different between the two versions.
I wouldn't say that it regressed at all! It is different, and it takes some getting used to, but it's actually quite a bit better IMHO...
The new Detached Display is much more powerful than MC11.1's method (it's now possible to have two different displays running full screen in different zones for example). You can even start a song playing in Zone 1 and detach the display, move it to the second monitor and full-screen it, then switch to Zone 2 and start a Video file (using the same sound card) and watch it full screen on Monitor 1, both at the same time. The detached displays feature lets you have as many independent, concurrently running display windows as you want, and you can control them all from the regular MC UI.
I would strongly suggest that new users to MC12 leave the Display Settings --> Full Screen --> Monitor setting to Nearest Monitor for a bit and try it out that way. It works quite intuitively that way and it's IMHO vastly improved over MC11.1's handling. Previous versions of MC seemed pretty clunky in that it forced you to choose which monitor you wanted your full screen display to live and basically always have it there (without digging into a menu anyway).
MC12 by contrast puts the Full Screen on whichever monitor the display is currently "living on" when you send it full screen, which is just easier! If you want it to go full screen on monitor 3, drag or send the display there (either by dragging/moving MC's whole UI, or if you want to be able to control it separately from a different monitor then use the detached display) and double click it. If you then decide you'd like it to go to Monitor 2, you put it there and double click it. If you find it too difficult to drag the window to another monitor, and you have multiple displays, you should really look into getting UltraMon (which has hotkeys and buttons to quickly send windows to and from monitors and is just awesome in so many other ways). Actually, if you have more than one monitor, you should really look into UltraMon anyway...
It seems simple and elegant to me, and as a DJ tool it's WAY more powerful (letting you have multiple detached displays is awesome)! Check out this screen shot of my desktop with two monitors, and MC running 3 different Detached Displays at once:
(Click on the thumbnail to view the full size image.)
I only have 1 sound card in this machine, and both the sound for the video and the music work at the same time. Also, any of those displays can be sent "full screen" on any monitor I want by simply double clicking on it (which will Full Screen it "in place" on whichever monitor it's already on).
In v12 the main windows jumps to display 2 and is used for full-screen display. If you change the display settings back to screen #1 at this point, the window becomes invisible and it takes 2 clicks on the task bar item to regain visibility (the first click minimizes the invisible window and the second click brings it back to display #1).
This doesn't happen for me. Though I no longer use the Display 1 or 2 settings in Display Settings myself, I just tested it and I'm able to switch back and forth between the two seamlessly. If I go full screen on Display 2, then right-click and choose Display 1, as soon as I click OK the display flips back to Display 1 as it should. I can go back and forth as I choose. Double clicking again returns the UI to where it was when I started, as it should...
One thing I would like (which I don't think is MC's fault so much as G-Force's) but now if you have one Detached Display running G-Force in Zone 1 and you open a separate Detached Display in Zone 2, and you start something new playing (even videos or images) each time the Zone 2 "track" changes G-Force in Zone 1's display "detects" it and shows the "track info" popup for stuff not happening in that Zone.