How can students with varied interests decide which 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.