I'm guessing it's a case of logistics and customer service. While the PC cruise might not be sailing at capacity, I'm guessing there will be at least 1000 guests aboard, if not more. That's still quite a few people to add to the island. Not only does that add to work/lines, I'm sure
DCL has considered the possibility that quite a few of the Dream guests might complain about the added numbers cutting in to "their" island time. At the end of the day, you can't make everyone happy. Since the PC cruise has never advertised a stop at CC as being part of the itinerary, and the Dream passengers assume they get the island to themselves, I can see where DCL might think this could cause some ill will with the Dream passengers.
As far as everyone getting on and off correctly... I can just imagine the nightmare of some kid getting back on the wrong ship and sailing off without his parents.

And there's probably all sorts of insurance and liability issues involved with using the tenders that bring the locals to the island for services. I'm guessing that's just considered their mode of transportation (like us driving to work in the morning) and is in no way associated with or insured by DCL.
For all we know, DCL did actually consider it, looked at the options, and decided it would be a nightmare to get the docking/tendering/KTTW cards/lines all working right. As far as I know, they've never had two ships unload on the island at the same time. But I could be wrong.