JIRA doesn't do "multiple simultaneous assignees", because that opens the door to "I thought someone else was doing it", which despite people saying "It won't happen to us", always does, in my experience.
So, bearing in mind that you should think of the assignee as the single person who currently has responsibility for owing the issue, there are plenty of good ways to show other people's interests in it.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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