My point was that when the Minnesota tax on a joint return is higher than the Minnesota tax on a separate return, then it means that Minnesota is either (1) taxing the Virginia spouse's income; or (2) taxing the Minnesota resident more because of the Virginia spouse.  There should be a law against it, either way.  If not the Constitution, then a federal law, like the one that says Minnesota can't tax pensions once its resident moves to Virginia.  

As a practical matter, I don't think Minnesota has an easy way to find out whether its resident is filing a joint return, when the address on the 1040 is Virginia.  IRS sends Minnesota a list of returns filed with Minnesota addresses, and Minnesota's computers rely on that for matching.  IRS does not send Minnesota a list of all returns filed in every state.