Mixing models almost assuredly does increase accuracy in many cases - I do it all day every day and I can only say from experience though not true objective benchmarks.

I use codex/claude code all day every day and built an MCP that allow them to use each other as sub-agents.

I will not allow claude code to touch my code with any design changes without codex auditing it, and visa versa.

I believe this is the case merely because they are different models that will provide varied perspectives, and smart enough to identify issues on each other's work.

I'm not sure if there's research to back this up but I think it's fairly obvious, and also FYI diminishing marginal return - I'm not sure if a '3rd perspective' would be worth it in most cases.