I’m working on the same thing right now and hope you’ll permit me to hijack your thread with a relavant question in a minute.
The technique that T. posted for setting custom resolutions works great (Thank you for putting that video together!) and was the best solution for my project because we have images and videos that stretch across 3 displays.
Couple things to keep in mind using the custom timeline technique:
Transitions occasionally get a bit screwed up when overlapping video that doesn’t actually overlap. The best fix I’ve found is to delete your render files and let FCPX re-render before exporting.
If you’re using stills, sizing them first in Photoshop to the dimensions of the screen(s) will speed up your workflow. When you import them into the timeline you can type 1920 or -1920 into X position to quickly move your clip to the left or right “screen.”
Even if you choose to use 3 separate video files, it might make sense to start from a single project instead of trying to get the timing right across 3. I’ve used a free program called QT Sync in the past to generate 3 files from a single master export.
My problem and what might soon be yours is that a 5760×1080 video takes a machine with a lot of horsepower for smooth playback. I’ve been exporting mine from FCPX using the “Export file” (H.264) option. If I try to send it to compressor any H.264 option at full size produces a file that is scrambled. Is there a better codec to use on a file this size? The lower the CPU usage the better. We’re using a Windows 7 machine running VLC for playback.
Good luck.