Check the rules, ins and outs, for both states. Snowbirds have similar problems, the states seem to deal with them.
My brother maintained his domicile in Michigan while a stick and brick snowbird in Florida. Two Michigan cars were tagged in Michigan, house maintained there and used June-August and a couple weeks in December. One car and street-legal golf cart tagged in Florida, kept at the house in Florida. Houses and vehicles were insured in the states where they were. Voter registration and drivers license maintained in Michigan.
Neither state had a problem with all this while he was alive, but Michigan will putting everything into a Michigan trust turned out to be a problem, two years after his death we are still working our way through probate in Florida to get the house into the trust. So watch details like that.
Another example, RV snowbird cousin has Michigan domicle, spends October through April at a park near Fort Myers. His RV gets stored in Florida, so it gets Florida tags and is insured there. Truck, SUV, trailer load of motorcycles he takes to Florida remained tagged and insured in Michigan, as are whichever motorcycles in his collection that get left behind every year.
But it is also about the two states. Note that both domiciled in Michigan, an income tax state, rather than Florida, which has no income tax. Why? Because it is easier, legally, to be a long term visitor or part time resident in Florida than it is to be domiciled in another state and be part time resident of Michigan.
So I don't know how Washington-California combination works out, but it might be easier to maintain domicile in Washington and be a long term visitor in California. That's if you are not working. Working and earning a living complicates things, but still, even California has a large population of guest workers.
