It all started with this tweet I wrote:
My brothers and their bizarre idiolect: if one brother doubts the other, the second brother will say, "Yo, Cyrano!"—i.e., "Yo, I'm serious!"— Kevin Kim (@bighominid) November 12, 2016
Fellow tweeter Barry White saw my tweet and chimed in:
@bighominid If more than one person does it is it still an idiolect?— Not a pundit (@ESLBarry) November 12, 2016
I'm aware of what "idiolect" means, and I didn't think I was misusing the term to describe two individuals who happen to share the same speech pattern. I replied:
@ESLBarry Maybe I should've just added an "s." Then again, if the brothers share the same idiosyncratic speech pattern... why?— Kevin Kim (@bighominid) November 12, 2016
But Barry politely insisted:
@bighominid I think when it goes beyond one person it ceases to be an idiolect. Sorry for the Saturday evening pedantry.— Not a pundit (@ESLBarry) November 12, 2016
I gave two replies. First:
@ESLBarry Hm. So even if 2 ppl have the same idiolects, we can never speak of "one idiolect" as a way to cover what both ppl R doing.— Kevin Kim (@bighominid) November 12, 2016
@ESLBarry Sort of like: you and I are writing Englishes, not English.— Kevin Kim (@bighominid) November 12, 2016
Perhaps you're not seeing my logic, Dear Reader, so let me explain.
The term idiolect actually has several definitions,* so there's definitely some semantic wiggle room—enough to make me think I didn't misuse the word and thus didn't need to be corrected. My point, in the replies you see above, is that (1) David has an idiolect, and (2) Sean has an idiolect, and (3) they converge/overlap in the instance of "Yo, Cyrano!" Why not just use the singular, then, to describe this situation?
You might say that an idiolect is the sum total of linguistic knowledge, tendencies, and actual utterances of a person, so David and Sean have idiolects—plural, countable—and their overlapping communicative styles can't be labeled idiolect—singular, uncountable. But if you insist on this position, you put yourself on shaky ground because that sum total is always in flux, thus making the term "sum total" vague bordering on meaningless. Conclusion: I can legitimately use idiolect to refer to only part of a person's speech patterns. I don't have to be constantly referring to the entirety of his or her speech patterns every time I use the word. Linguists, after all, do this all the time in discourse analysis.
Does it make sense to say that "David speaks English, and Sean speaks English, and when they get together, they speak Englishes?" Only if you're of the school that contends that idiolects are all there is, i.e., there is no communal, generally accepted version of English: there is only English as it's instantiated from person to person. This is analogous to the line of thinking that says, "Even though we use the term language, there are only languages: language-in-general doesn't exist. I speak French; you speak Spanish; neither of us speaks language, per se."
And yet the general, unifying term language still exists and has meaning as a singular uncountable noun. That was the sense in which I used idiolect in the singular.
You might say, "It's safest to call what David and Sean are doing a dialect because it's a speech community of two." So let's say Sean drops into a black hole: only David remains. Is David still speaking a dialect, or is it now an idiolect? What about the valley village of Ours-sur-Marne, which has its own distinct, growly, ursine French dialect? After an avalanche kills everyone in Ours-sur-Marne except one lone villager, is that villager now speaking an idiolect, or do later researchers still identify his speech pattern as a dialect? "Jacques is speaking a long-lost ourssurmarnien dialect," or, "Jacques is speaking a long-lost ourssurmarnien idiolect"?
My point is: terms like idiolect and dialect seem rather context-dependent, so I don't think it's obvious that, given that I was talking about individuals, I was wrong to use idiolect in the uncountable singular, the same way we'd say "Bill and Ted speak English," not "Bill and Ted speak Englishes." There's no justification for a plural.
I'll give the final word to the Stanford Encyclopedia of Philosophy, which has this to say on the matter of idiolect:
Although the properties of x's idiolect are tied stipulatively to intrinsic properties of x, this does not mean that two distinct individuals could not share an idiolect in principle, or have significantly overlapping idiolects in practice.
And that, folks, is all the expert validation I need to prove my point. (In fact, that's really all I should have written!)
*For me, it's enough to think of an idiolect as "a dialect of one."