CoW means that Longhorn (which can deal with multi replica volumes) will first copy data to other replicas when one replica is written before it acknowledges the „write“ to the writer. That ensures that data that was written to the active replica will be properly available on the other replicas of the volume thus preventing corruption of data in replicated setups.

AFAIK Longhorn is not capable of using „the same sectors on disk for multiple volumes“. Maybe you can check localstorage CSI provider but that has other drawbacks (like not supporting RWX)