Yup - if you only intend to play 2 characters, then you can easily just play using one PC: whether you play with dual monitors or 2 clients in 1 monitor.

Your next option would be whether you'll be using AHK (Autohotkey) or the basic alt tab approach (or have the windows small enough that you can drag the mouse easily between wow window clients).

Xzin and others make a very solid statement that at some point, Blizzard may bring the hammer down on AHK users. But for now, with just basic keystroke being sent to the background/foreground wow client - it's "safe."