Okay now that I have completed the CPL (single) and am about to take the multi, I think I probably made a mistake.
You can take both tests at the same time. The multi will count towards the 15 hours of non-instrument time, which you can complete on a cheaper PA28. The multi rating requires you to perform all of the same manoeuvres as in the CPL except basic Nav and instruments which is simple enough. Plus the multi is more stable and arguably easier to fly more precisely.
So if you are doing both, personally I would combine the two. I wish I had not logged an extra 10 hours or so in PA28s when I didnt really need to.