The explicit requirement to have only one owner for one task is deeply embedded in Phabricator's design. I'm certain upstream will not accept any patch changing this behavior. In fact, the Wikimedia Foundation projects are going in the same direction of identifying single owners for tasks and projects, and the history of Bugzilla (and therefore most of the community expectations) are just the same.\

You can create subtasks to identify which chunk should be driven by whom, and you can change assignee when the same task changes of driver. CC fields and editable description should be enough to rack and document anything else.

Unlikely to happen and I don't consider this helpful at all.
If there are use cases, please provide use cases. "Because other software does" is not a use case in itself.