My hospital experience is a little bit different from you guys. All I did was IV. We were supposed to rotate, but for some reason, we never did. I did them faster and better than the pharmacists working at the time because they never made anything. They did it once or twice when it was some really expensive drug, but that was it.
In my opinion, it doesn't really matter what hospital you work. No experience is a bad experience. You will learn something regardless. Besides, I don't think it's wise to work at 2 different hospitals at once. But all you really have to do is be honest. Tell him that you can't start until so and so date. He is not going to ask you why and even if he did, just be honest and tell him and explain your reasons. There is nothing wrong with a hard working student wanting to learn as much as he wants, the only problem the interviewer might get out of this is that you will not be flexible due to another job.