knock (people's) heads together
To forcefully make two or more people act, behave, or think in way that is appropriate or necessary. We're so far behind because your team keeps procrastinating. You need to go in there and knock their heads together. She claimed she would go into the negotiations and knock everyone's heads together until a deal was reached.
knock your/their/our heads together
To scold two or more people harshly. If you two do something this stupid again, I'm going to knock your heads together!
knock people's heads together
or bang people's heads together
mainly BRITISH, INFORMALIf you knock people's heads together or bang their heads together, you forcefully make them stop arguing. If he's unable to knock everybody's heads together, then questions are going to be raised about his own ability to continue in office. The idea is to bang their heads together until they come out with a sensible deal. Note: You can also say that someone bangs heads together or knocks heads together. Chris Smith said he would bang heads together.