Using Single-Splitter as a base, this simply re-colours it, and makes it swap the two sides of a belt. It is pretty simple, and requires a belt on either side (so can't feed directly into or out of anything else). It also has render issues with the belts overlapping it (though that doesn't change how it works). There's also no animation for it, simply a static little belt-swapper that saves some space.

This does enough for me, so not got the time to spend learning how to improve it to bypass those issues.