My advice, Steve, is dont waste your money flying twins until you get your CPL. Use the money for private hire and get some experience in high performance singles. At PPL level, you will not have the depth of experience to get full value from ME training.
CPL training is really just PPL stuff pitched at a higher level. Use the private hours that you fly to do some 'real' flying. Plan to go crosscountry, to remote areas, in contolled airspace etc. and properly research and plan out the flights before you do them. Make sure that you know all the AIP, CAO, CAR requirements of any flight you undertake. Treat each one as a training exercise.
Once you have your CPL by all means get a ME rating. However the full benefit of ME training comes by doing it as part of IR training.