I would look to Automation to create something that can spot a change of priority and poke a value into a custom field. Or set priority based on a custom field.
But, I'd question the need for it. A described, this just feels like you're duplicating the priority field for no good reason other than you think you want a severity field. Could you tell us why you want to do this? Or is there a limited set of circumstances in which you want to do it (for example, if the user forgets to set a severity on creation, assume it based on priority)
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