Aura Seltzer, writing for Typekit:
Focus on the backstory of each typeface — the context for its creation. You can combine typefaces that were informed by the same tool, output medium, historic era, or concept. For example, you can pair typefaces that were both inspired by calligraphic brushwork, were both designed for low-resolution printers, were both designed in the early 1900s, or were both conceived to address legibility.
Paying attention to history is one of several approaches Aura takes in this study about combining typefaces.