There's a vague guideline: contrast in some aspects, lack of contrast in others. You want your reader to be able to smoothly read on, but you want the difference in typeface to be clearly noticeable even to a casually observing layperson.
When combining two typefaces in a single line, make sure that there is enough contrast between the two:
- Make sure the two typefaces are from different categories, like sans vs. classic serif. If the regular body text is a classic serif, don't choose anything that might be mistaken for a classic serif as a second typeface.
- In most cases, even then it's wise to emphasise the difference between the two faces by using a contrasting font for the second typeface so it really stands out. Think bold or black/extra bold.
When combining two typefaces in a single line, make sure that the reader can smoothly read the line:
- Ensure that font x-heights match—that's the height of the lowercase letters. That doesn't just mean to match up the displayed point size, but also eyeballing the lowercase letter sizes and check whether they match as much as possible. Some typefaces at eg. 12pt are significantly larger than other faces at the same 12pt. Compare Garamond and Verdana for a good chuckle: even Verdana's capitals are significantly larger at the same point size.
- Do not change font colour in addition to changing the typeface and weight. You are already drawing attention to the difference with the typeface change, no need to hammer it home.
Lastly, there's of course an aesthetic aspect: choose a second typeface that combines well with your first. There is loads and loads of guides for this around, both online and off.