WES is not mandatory for your spouse as long as you are not summing up your both scores. It would be great if both have IELTS and WES so that you can get more points from it and increase overall score
It is optional, if you do you can claim up to a maximum of 10 points for your spouse's education, but it cannot be combined with your WES assessment, it will be an altogether new assessment application with WES which means additional cost. In the end it is your call, you can look at the benefit vs cost.