I've always had difficulty ascertaining the way to approach structuring a particular type of sentence re: the situation in the question title.
There are probably all sorts of ways to restructure this particular example in such a way so as to avoid the issue entirely, but unless that is, in fact, the general rule of thumb in these types of situations, I want to make sure we focus on the specific concept of dealing with two prepositions of this type rather than just dealing with the no doubt poor example. As follows:
"...the value of which, I might add, humans are completely delusional about."
What I want is to stick that "about" somewhere that is not the end of the sentence. As I have done for "of", I know we often use "which" to make sure that our prepositions don't end up at the end of our sentences.
In the above example, if it was "the thing" that humans were delusional about, it would be easy to just end it with "[the thing], about which humans are delusional." But instead, it's "the value OF the thing" about which humans are delusional. So if I wanted to structure the sentence more or less like I've done, coming at the end of a larger monologue, as sort of an afterthought, I'm not sure how to squeeze the "about which" in there with the "of which" that's already there.
For instance, could I double use the same "which" somehow? Or must I slide a second "which" in there to use with the "about"; if so, in what order should it go, exactly?
Note: I realize that with this example, I could easily just add the words "is something", so as to say:
"...the value of which, I might add, is something about which humans are delusional."
But again, I want to focus on dealing with this concept in all contexts, rather than avoiding it here just because it happens to be easy. (Also, even that sounds a bit repetitive to my ear, what with the two "which"s so close to each other. The "I might add" is really there to try to add some separation but it isn't all that effective.)
This has been on my mind before more than once, and, if I recall correctly, I'm pretty sure I once heard Frasier Crane solve this for me with some fancypants word magic in exactly such a situation, and it sounded great. I want to say he somehow doubled down on a single "which"; something like "of about which". The problem is that that doesn't really seem right to my ear when I try it now, but I can't for the life of me remember or figure out how else he might have done it.
Thanks in advance to anyone who will help scratch this itch.