If your dualing 2 of the same class, then yes I'd operate both from the same machine/interface.

For dualing different classes (e.g. Priest/Warrior) I'd operate them seperately, and did so with my priest/warrior from level 10-70. For me it's somehow helpful to have them separated mentally. Also, if you go with this suggestion (and it matches what you're doing) I'd highly suggest getting something like a Belkin N52 gaming pad (check Hardware forums for options) as they're designed for one hand use, vs. a standard keyboard... plus they have a smaller footprint, and they're cheap (~$30).

GL&HF!