Watchout controls the playback of sequences and keeps all the projectors in sync, but the clips are split first.

Typically these sorts of things begin with a staging company designing the set and working out the physical size of the screen (or multiple screens).  The physical size is then used to determine how many projectors are needed and where they will be placed.  Projectors are run at their native resolution, which varies between models.  1920x1080 is increasingly common, but there are still lots of 4x3 projectors that run at 1400 x 1050, and other sizes too.  Allowing for overlap, the staging company will come up with an overall screen size in pixels, and details for how that canvas is to be split into individual streams for each projector (always the projector's native resolution).  Areas of the screen can be masked as well to suit the stage design, so some of the numbers you get can be unusual.  The canvas I'm working on for this job is 11,431 x 1080 and this gets split between 7 projectors, each 1920 x 1080.  There are another 4 projectors  for the floor area, which is animated separately.

I'm not a watchout expert, but I think each stream is played by an individual computer.  At least they used to be, maybe faster computers and SSDs have changed things in recent times, but I'd be surprised if they risked playing more than one stream from any computer.  So for this project, which has 11 projectors in total, I think there will be 11 individual computers, each playing an individual stream to each projector, sync'd to a master computer (server) where the watchout programming is controlled.  It's kinda like powerpoint on steroids.

On smaller sized screens, I have worked on projects where there was only one large video file and the playback from a single computer was split between multiple projectors by a spyder.  But with this type of scenario you are limited to movie sizes that the computer can reliably playback.  With SSDs and faster machines this can be pretty big, but it wasn't that long ago that anything larger than HD was risky.

-Chris