How can students with varied interests decide which to pursue in graduate school? I am someone who has wide-ranging interests, and these interests are not terribly interdisciplinary. (Say someone is interested in Physics and History. Or perhaps, Economics and English. Or, Civil Engineering and Music Composition. You get the picture.)

The fields I am interested in do not inherently intersect. They can be fused, however I don't think this interests me, because it would result in a "perversion" of each respective field, for lack of a better word. It seems forced...and would probably ruin my interest in both fields.

It's not that I "like" one field more than another; both fields are interesting to me and I cannot calculate which one is more interesting. The fact is, they are very different, and I like each one for different reasons. 

I want to go to grad school, because I enjoy learning, reading, writing, etc. and I am interested in a career in academia. But I am trepidatious about immediately going into grad school because I basically believe, "Whichever field I end up choosing is the field I will be destined to spend the rest of my life (or at least many years) exclusively studying. I really don't want to mess this decision up."

How can someone in my situation decide which interest to pursue in graduate school?
If you like two subjects equally, pick the better paid one for your actual career. That one should be the basis for most of your formal, documented education.

You can still treat the less well paid one as your hobby, and go for informal education in it.

Taking your first example, Physics and History, I would make Physics the subject for formal education such as a master's degree. Go on reading about history, join a historical recreation society, take on-line courses in history, blog about history as your hobby.