To add to this - even in a monorepo, a single agent hitting a 200k+ line codebase gets slow and loses focus.
The multi-agent approach lets you scope each agent to what it knows best. Backend agent stays in /api, knows the DB schema cold. Frontend agent lives in /web, knows the component library.
When they need to coordinate, they message each other with just the relevant context - not the entire codebase.
It's also useful for parallel work. Both agents can be coding simultaneously instead of one waiting for the other.
personally - and maybe I'm biased, I prefer the expert approach.