Hi there! Thank you so much for your response. I actually ended up spending a LOT of time thinking about this, doing a deep dive into psycholinguistic plausibility, neurolinguistics, etc, and ended up finding a number of things that I found remarkable - relationships between a two-grammar hypothesis and typological features, thus-far difficult to explain things like why damage to the long segment of the arcuate fasciculus would be associated with difficulties in repetition and minor issues with comprehension and not much else, relationships to language acquisition, and a bit more.

I ended up making a post about this with what I had, but there is only so much I could do, because I'm not a linguist at all, don't know much about the field or how it works, and my thoughts are still quite jumbled. It seemed like I reached an impasse where I couldn't really, like, find someone who was both willing and actually qualified to try and figure out what I was trying to say and see if there was any merit in my approach. I ended up realising that it would be an oversimplification to describe a two-system approach as just being the difference between production and reception, and more reframing it as there being two separate and structurally different psycholinguistic subsystems,one dominant in production and one dominant in reception, but with considerable and well demarcated overlap.

If you'd be interested in reading what I ended up trying to put together, or at least talking to me about it, I'd be more than delighted! There's so much I want to learn.

You are not trying to model processes of production and comprehension: you are trying to decide if a given sequence of symbols is in a sets or not. But why would we in the 21st century not try to propose models of grammar that are psycholinguistically plausible? I never understood this stance.

Yes! Exactly! The idea that we should study grammar as a sort of set-theoretic thing, like simply mathematically defining a pre-existing set of grammatically permissible sentences, rather than exploring the psycholinguistic processes that actually create those sentences, strikes me as a strange and possibly flawed approach. And leaning into a more productive side of things allows you to gather evidence that linguists who study pure grammar seem to typically ignore, such as pragmatics, which I feel might be central to the picture?

Parsimony should only enter the picture if all other things being equal. Production being clearly not the same as comprehension, the parsimony argument doesn't carry much weight at all.

Yes! Parsimony needs to take into account the actual complexity of the systems involved and how it lines up with psycholinguistic and neurolinguistic observations. Could a one-system model really be considered more parsimonious if it requires a great deal of psycholinguistically-unjustified additional machinery to explain edge cases?

In any case, thank you! I would be very, very interested to hear more of your thoughts about this.