Employee working on multiple state in a week

We have one more interesting requirement.  

Employees who are in Washington once in a while go and work in California at client site for few days.  Employer have to pay tax for the hours employees worked in CA. I am thinking only way to do that is by updating IT208 for those days employee worked in CA. Is there any other alternative approach?

Also, we have  to pay WEEKLY overtime proportionally between WA & CA.  When it comes to time eval, not sure how we can know when the employee working in CA or WA.

I would really appreicate if you have any suggestion on the above requirement.

Thanks!

Jesu