I rarely find myself wishing to split my screen. It's not that I don't know that I can, and occasionally I will (especially if two pieces of code happen to be VERY tightly coupled, with lots of passing back and forth between the two). In general, however, I find that since I am only reading one line of code at a time, having two editor windows up is pointless. It's like having two browser windows instead of two tabs in a browser window.

Plus, with most editors that can jump in / out of functions, it's not like I'm losing my place. In fact, my editors all save my place in files on exit.