Enthusiasm for fonts often leads to enthusiasm for multiple fonts, and then the question:
Mixing fonts is like mixing patterned shirts and ties—there aren’t blackletter rules. Some people have a knack for it; some don’t.
Keep these principles in mind:
The rule of diminishing returns applies. Most documents can tolerate a second font. Few can tolerate a third. Almost none can tolerate four or more. (If you’re making a presentation, consider all your slides to be part of one document.)
You can mix any two fonts that are identifiably different. If you’ve heard that you can only mix a serif font with a sans serif font, it’s not true. On the contrary, much like mixing colors, lower contrast between fonts can be more effective than higher contrast.
For instance, you wouldn’t mix Palatino and Palatino Nova (see palatino alternatives) —the differences are too subtle. But I would consider mixing Lyon and Harriet, even though they’re both serif fonts. For inspiration, look at any American newspaper—typically, the body text and the headlines are both in serif fonts, but different ones.
Font mixing is most successful when each font has a consistent role in the document. In a research memo, try one font for body text and one font for headings. Or in a motion, try one font for things in the center of the document (body text and headings) and one font for things at the edges (line numbers, footer, and other miscellany).
It rarely works to have multiple fonts in a single paragraph. Better to restrict yourself to one font per paragraph, and change fonts only at paragraph breaks.
Though I’m typically reluctant to endorse rote methods, this one works reliably: combine fonts by the same font designer. For instance, Harriet and Alright Sans, both designed by Jackson Cavanaugh, would mix well. For that matter, so would Concourse and Equity, both designed by me.