Thanks for the reply and the links. 

The client is CA resident by domicile (clear intent to return to primary home after job assignment is completed in ?? months/years), and ID resident by days-in-state.

My client *might* qualify for the safe harbor next year (needs 1.5 years out of state with less than 45 days visiting CA), so I think my best bet may be to file CA as non-resident anticipating that this will be the correct status by mid-2022, and advise the client not to exceed the 45 day limit returning to CA in any given year.

But from a program perspective, I'd rather not have to rely on the safe harbor.  Filing dual-residency isn't that much of a burden since there would be a state tax credit to wash-out any double-taxation.  But I'm not sure this is the year I want to be submitting returns on paper just to take the most prudent approach.