looking for a hdmi switch that will let me split screen output to a projector

In general, a switch lets you select among several different inputs, so that one of them goes to a single display, which can be a projector or a TV.

Your question is rather ambiguous, though.

Do you mean you want to view two separate images with the same projector at the same time? For that you'd need electronics which supports PIP (Picture in Picture).

Or do you want to view the same image on two different displays at the same time, one of them being a projector? For that you would normally use a splitter.

If you want to select from several different sources and send a different image to each of several different displays, you'd need what's called a matrix switch.

A Web search for "split screen video processor" located quite a few.
e.g.
http://www.ambery.com/spscvipr.html

I don't have any experience with any of them.