As a Kanban user, I would like to set a single WIP limit that applies to two (or more) columns so that I can have more flexibility in how I allocate my resources and can achieve a smoother flow of work.

There are many examples online of Kanban board having "Doing / Done" columns for "Development" or "Testing" and most of them have a SINGLE WIP limit that applies to both of those sub-columns.    Giving Jira the ability to apply a single WIP number to more than one column can significantly improve the flow of work.

For example you could have 2 tickets in "dev-doing" and 2 tickets in "dev-done" all adding up to 4.

Or you could have 3 tickets in "dev-doing" and 1 ticket in "dev-done" adding up to 4.

In the most general case, you could set a SINGLE WIP number for the entire board.  This is a nice gentle way to introduce WIP limits.  You could tell the team-- no more than 20 cards on the entire board.