I had the same exact experience as you did and based upon my phone inquiry they (the CIC telephone agent and his supervising officer after putting me on hold) told me that you can have two valid permits and the conditions of one, do not invalidate the other. However, I wish I had written confirmation from them i.e. an email, because I was recently talking with an immigration lawyer and he was so adamant that you cannot work more than 20 hours even if you have both a PGWP and a study permit. Tbh, it has caused a bit of distress hearing how sure he was, but I still have more faith in what I was told by CIC, plus the majority of my work was on-campus and related to my studies (RA/TA) so hopefully, that wouldn't be a problem