FFF has it right in terms of multi rating and CPL licence. You can use some of the CPL mandatory training hours to do your multi training, and take both tests at once... but you will probably have enough stress getting it right on a single engine, than worrying about multi-issues too.
Comments about why do you want multi rating are also valid.
FYI I just completed CPL on single, and am now doing multi rating. Will then do some hour building on the multi to nail it down.