As far as I understood it, the CPL, being a Licence can be done in a single and the MEP, being a rating, can be added to it as you would a PPL.
I never saw the point in doing the CPL in a multi so maybe I am wrong but to do that would likely cost more than doing the MEP rating separately to the CPL in a single wouldn't it?
Unless you have access to a multi engined aeroplane at a good rate, in which case I can understand it.