Wash. Rev. Code § 4.08.040
Either spouse or either domestic partner may join in all causes of action arising from injuries to the person or character of either or both of them, or from injuries to the property of either or both of them, or arising out of any contract in favor of either or both of them.
If the spouses or the domestic partners are sued together, either or both spouses or either or both domestic partners may defend, and if one spouse or one domestic partner neglects to defend, the other spouse or other domestic partner may defend for the nonacting spouse or nonacting domestic partner also. Each spouse or each domestic partner may defend in all cases in which he or she is interested, whether that spouse or that domestic partner is sued with the other spouse or other domestic partner or not.
RCW 4.08.040
Part headings not law-Severability-2008 c 6: See RCW 26.60.900 and 26.60.901.