You do not. Because that way, madness lies. You should never never never have more (or less) than one person as the person ultimately responsible for an issue.
It is of course, absolutely fine to have other people who can monitor, be involved with, and take over from the assignee if they're not around. There are many ways to do that - add a custom field of type multi-user picker, or a group picker. Use watchers, or add a custom field with teams listed as selections.
No, you do not solve this problem with multiple assignees (in fact there are no problems solved by multiple assignees - it's actually a problem in itself, that would cause you a lot of pain if you could do it)
What you mean is that you want to have a group responsibility for picking up an issue and, probably, have the ultimately responsible person selected from that group.
The obvious way to do it there is to have a group picker for "responsible group" and set the permissions such that "assignable user = group named in responsible group". You leave the assignee blank (unassigned) until the actual assignee is chosen. Or use watchers or other fields, or or or.
Or, if you are in a Service Desk project, use the Queue function as intended - the issues in a queue are effectively assigned to all the agents in the project.
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