Although you rarely hear about this feature, it’s been around for years, probably close to a decade. Panasonic may have had the first projectors that could split their screen and place the screens of more than one PC up on the projected image. Epson and others also offered a projector here and there with that same type of feature set.
At this point it doesn’t really matter who was first. It’s not a feature, apparently, very widely used, but it certainly is a feature that offers great benefit when used effectively.
The traditional approach to Multi-PC sharing would be for the projector to receive four separate sources (usually by wireless or wired networking), and put one in each of the the four windows the projector produces. This way a teacher, for example, could put up the work of 3 students, along with some relevant notes the teacher wishes to show for perspective. Or, it could be used in an office environment for collaboration between four team members so that the group can see multiple points, ideas from the members, displayed at the same time. Think about “your world”, and if you work in groups, or with groups, you’ll probably be able to quickly see some ways where having multiple computers displayed at once is of benefit.
In the case of this Epson projector, it is capable of displaying up to four screens simultaneously out of a collection of about 30 computers that are tied in.