There's some real research-backed evidence that this approach elevates output quality, and it was even discussed a few months back: https://news.ycombinator.com/item?id=47169627

Anthropic confirmed it in their recent report https://news.ycombinator.com/item?id=49316271

> We expect that agents coordinating in the wild will act in higher variance ways than we see here, because they’ll have different backgrounds and therefore different contexts. They also, presumably, won’t all be Claudes.

I do use a second Claude (both Opus) to review the first one's output - it almost always finds worthwhile (often critical) observations. They have different 'perspectives', one wants the job done, one wants to find problems, is how I had it explained.

Claude chat itself called it 'Claude-on-Claude action', which I found cute.