Ms. Kitty, this is certainly doable, but with two weeks, why press yourself?
I've done the ruins and, while I am glad I did it, wouldn't do it again. JMHO, and again, I'm glad I did it. Manatee's are cool and any operator that stresses the need to preserve these magical creatures is worth patronizing and there are some good ones with good trips.
Now, as to Placencia, I'd highly recommend that you give yourself a couple of days there, especially if stalking Whale Sharks. We spent a week or so in the southern part of Belize and loved it. I have not done the Whale Shark thing, but believe me its on my immediate list of things to do. If I can get up close and personal with one of natures most magical creatures, I can die happy. That said, they aren't always found on a given trip, so why not spend an extra day and double your chances. There are great places to stay and great people to meet.