Ah, it's not quite that simple. There are an unfortunately large number of plants with effects that are best or exclusively cured through dual cultivation.

[Long suffering sigh. Some god clearly had a very, very bad sense of humor.]

An energy transfer between partners. Including, it seems, a few that may require a specific breed of demon to be involved.

[Wasn't told that they cured without-a-cure with dual cultivation, but Shen Qingqiu had it before he married Luo Binghe, and didn't have it after. It's an easy conclusion to come to. Would love to pin them both down and question them about it for hours.]