My son is married and attending college in Idaho. He worked in Idaho but also California during the year. His wife also worked in Idaho. What state should they be residents of? They are not planning to stay in Idaho after 1 more year of college.
Did they have any permanent (in the sense that they might have returned to it) home in a state other than Idaho? Where did they live most of the year? Residency is determined by multiple factors. Here is how Idaho defines it: https://tax.idaho.gov/i-2015.cfm
You are a resident of your state where your drivers license, voting records, bills, banking and normally where you plan to return home. CA has stricter laws than most states and will attempt to tax all income as resident income. They could in fact be residents of 2 different states since you say neither plan to live in Idaho.