Two fonts can be enough to design a poster, banner, or social media visual, but their roles should be clearly defined. One font can handle the headline, while the other can take the body copy, additional information, and the occasional small callout. The point is not to seek the prettiest combination, but to make the text easy to read and the layout feel clear before the reader even reaches the image or the color palette.
One way to start is by deciding the role of each font before you choose each font. Ask what role the headline needs to fill. Does it need to look powerful, serene, cute, fresh, or assertive? Ask what role the body copy needs to fill. Usually, this text needs to remain legible at a smaller size, and not compete with the headline. If both fonts are vying to be the star, the result will feel messy.
A powerful pairing often comes from contrast, though contrast does not equal chaos. A chunky geometric headline works nicely with a plain, restrained body typeface. Or the headline can be a serif font and the body text a clean sans serif. More often, problems arise when the fonts chosen look too much alike, yet just different enough to look accidental. If two typefaces are similar in shape, similar in weight, similar in mood, it is often wiser to pick one typefamily, and generate contrast through font size, weight, and leading.
Another exercise is to ask yourself if you can pull off the design by using one type family before deciding a second font will be needed. Use a heavy weight for the headline, a normal weight for the body copy, and play around with the leading so the block of text has more white space. Adjust the scale before adjusting the style. Make the headline bigger, adjust the tracking when it looks like you need to, and make sure the body copy has enough leading. It is a great way to see if the design needs a different typeface, or if you simply do not need to manipulate the type as much as you think you do.
As another exercise, take one simple layout of a poster with a headline, a single image, and a paragraph of text. Design it in three ways. First, design it with only one type family using different font weights. Second, pair a bold headline with a simple body type. Third, use two fancy fonts. View the three versions at full size, and then zoom out a bit. Notice how one feels easier to read, and how one is less likely to feel like the message needs to be deciphered.
The surrounding white space of type is as important as the font you pick. A good pairing will still look bad if the headline is not given enough room from the body copy, if the leading is too close, or if a text block is crowded against the side edge of the document. Check your margins, alignment, and distance between different text areas before thinking your fonts are the problem. At times it is the fonts you love, but the layout does not have enough space to allow them to shine.
One quick indicator of a good font pairing is if you can articulate why the fonts work. The headline gets attention, the body type is legible, font weights create hierarchy, and line spacing makes the text block look orderly. When you can see the reasons on the artboard, the design looks less like something you made up and more like something you intentionally made.