I tried that with my PIII, last year (we have a thread going on with is), and the conclussion was that it will take double of time when processing 2 WUs at the same moment, so is you divide that by 2is the same as processing only 1, but like Conjur said it only aplied to i processor, now if you have 2 processors system, that is diffeerent.
Best regards
Eltano