How to assign "value" of custom field to another existing field on a transition?

[Karan Nagrecha]

I have a added a custom field of "Next Assignee". Now when a task is moved from "In Progress" to "Resolved", then the member of "Next Assignee" should be updated in the "Assignee" field of the task.

My ultimate goal is following:

I have 2 members - say A & B, both develop and review code. Now if A has developed then B reviews and vice versa. 

So if A moves the card to Resolved, then it should be automatically assigned to B. And if B moves it to Resolved, then A should be assigned.

Please help me with this issue. Thanks.

[Michael Wolfe]

You can most likely accomplish this by using Automation for Jira.  You would need two rules that when the issue transitions to "Resolved", it checks the assignee and changes the assignee to the other user.