If I understand your request here, you are looking to be able to add a second Assignee to an issue in Jira. In native Jira, this is not technically possible to do. Assignees are designed to be a single individual intentionally. It is possible to add a custom field in order to include another user, as Nic has suggested, but this is not technically an assignee.
We do have an older document called How do I assign issues to multiple users that might offer some additional insights here and techniques to address the source of the request here.
Perhaps you could utilize a 3rd party plugin to Jira, such as Groupee Group Assignee. This add-on appears to be able to let you assign Jira issues to a group of users instead, which might be more of what I think you are looking for here.
I hope this helps.
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