Actually with discharge one screen can do it
So says an expert at discharge.

One who is inexperienced at both separating (or more so adjusting for) and printing a 1 color discharge print...can FUBAR a 1 color discharge as easily as a 3 color grayscale halftone print.
For the every day average joe printer guy, (two) is better in most all cases. So says me who does seps for a wide variety of skill levels for a wide variety of printing methods.
I've seen the results of some people trying a 1 color photo in discharge that is already inexperienced at printing halftones in the first place. It can be a long trial and error process.