The spirit of this license term clearly is to prevent something like asset flipping where you sell something that's essentially already available elsewhere for free, without adding any value of your own. (The linked Wikipedia article is about real estate flipping, but this behaviour is also rampant in Unity game development.)
Even though the license does prevent selling the font directly, it does still allow you to use the SIL-OFL licensed fonts in a commercial context – so it is clearly a free license (free as in Free Software Definition).
The OFL does not explicitly require that the fonts can only be sold together with other software, though that would be the typical use case. The second paragraph does explicitly mention bundling with software with regards to notice requirements, but this doesn't preclude bundling with non-software.
If we interpret the license very literally, we can observe that:
- you could sell two separate OFL-licensed fonts together.
- the notice requirements do not apply if you bundle or sell the font with non-software.
- selling the font alongside an artisanaly crafted lowercase ASCII letter
a in a textfile would satisfy the license.
However, overly literal interpretations are unwise. Legal documents like contracts or licenses should be interpreted in good faith. (To which degree this matters depends on your local jurisdiction, e.g. good faith principles are very relevant in many civil law and common law jurisdictons, except for the U.K.). Under the principle of good faith, if the intended meaning of the document is clear from the way it is written, but the intended meaning is not expressed 100% explicitly, then it is still the intended meaning that matters.
Therefore my interpretation of the OFL is:
- You may sell the OFL-licensed font alongside anything, including non-software.
- The spirit of the license is that in such a case the main value of the sale should be provided by that other thing and not by the OFL-licensed font.
- The letter of the license allows this other thing to be trivial and worthless, but that use of the OFL-licensed font might no longer be in good faith.
Of course, selling the font just means selling a copy of the font, and this copy will still governed by the OFL license (see paragraph 5). The sale cannot impose any other license on the font, so it cannot provide extra permissions and can neither impose any extra restrictions.