"But that doesn't answer the question. Half the husband's WA income belongs to the MT wife."

MT filing separately means you can keep the husband's W2 in his column only. That's why I asked the question, not yet answered, if all that separating had been done, because perhaps now, the MT tax is 0 for the husband. Maybe there is nothing computed? Or, maybe he is considered to have earned a share of joint holdings, such as investment income? Or, they can split some deductible expense, such as HSA? But not individual IRA. Etc. Until that is reviewed and rebalanced, we don't know if there still is MT tax for the non-resident.

All we can do is offer suggestions, since we cannot see it for ourselves.