The request for having more than one assignee has been made a couple of years ago (see links). All of them got rejected, due to the policy, that Jira sicks with 1 assignee.

My request is not to break this, but to add a functionality to the agile board only, that for example, the username behind an avatar is part of a user picker (multi users) custom field. The assignee can stay as it is (first drag&drop of avatar will fill assignee, next goes into custom field).

During the past years, working as an agile coach, I have seen many times that teams, working with a physical agile board, mark issues with multiple avatars (magnetic white-board, post-its and individual magnetic avatars).

It absolutely make sense, because the whole agile movement is all about "individuals and their interaction". Reviews, knowledge transfer and pair programming (XP) are best examples for marking an issue with more than one worker. Not only the solo work on one issue.

Implementing this feature will make Jira Software come very close to the physical boards.