From Longman Dictionary of Contemporary Englishknock (somebody’s) heads togetherknock (somebody’s) heads togetherinformalANGRY to tell people who are arguing that they must stop and behave more sensibly None of them can agree and it needs someone to knock heads together. → knock
Examples from the Corpus
knock (somebody’s) heads together• If you two don't stop yelling at each other, I'll knock your heads together!• He caught hold of the two nearest him and knocked their heads together, kicked two more and grabbed the ringleaders.• She'd just box a few ears, knock a few heads together like she did with the Rattries, and chase them off. 