At work, I currently have 4-5 different PM's on 4-5 different projects. Each of them has no idea what the others are asking, and I am constantly bombarded with one off tasks via hipchat or in-person requests. The point is not to complain about work, but to agree that management would do well to recognize the importance of process scheduling and queueing, and that a failure to do so is only detrimental to their own objectives.
I don't have _that_ many PMs, but when I have multiple people asking me to do simultaneous tasks, I email all of them, asking them to prioritize the list for me. I'm polite about it. And it mostly works out. But I never, ever shy away from doing this.