e.g. at some point (I don’t know if this is current) Horwath Clark Whitehill audits PwC, Deloitte by Grant Thornton and KPMG by Grant Thornton (? might have changed), KPMG audits Accenture (the former consulting arm of Arthur Anderson)
Something to bear in mind though, is that although in the UK, for example, LLPs must publish financial statements as do companies, there is no requirement for partnerships that do not have limited liability to publish financial statements.