I've done the opposite, moving multiple tightly coupled repos into a single monorepo. Saves the step of the llm realizing there's a bigger context, finding the repo, then also scanning/searching it. Especially for fixes that are simply one line each in two repos.
I'm a fan of the monorepo in general, even before LLMs. If using git it leverages git's best feature IMO, the commit as a snapshot of the entire repo. I've worked on so many projects where tightly coupled things are split across repos because it's thought of as a best practice, and it just makes it more difficult to figure out what code you are running.