You can't. Well, you can set up :autocmds
to toggle the Conceal
group's highlighting depending on the current buffer, but it would still be off for non-active visible buffers of the other type. In general, highlight groups are global, and that is unlikely to change.
To solve your dilemma, I would choose one highlighting for Conceal
that is both unobtrusive enough for your C/C++ use, yet visually distinct to show the replaced HTML entities. To me, completely clearing the Conceal
highlighting is not an intended use; it should always serve as a reminder that what you see is not what's actually in the text.