You can serve two sets of papers on either one of the spouses as long as you serve them at home and they both live there. In fact, you can serve the papers on any person of suitable age and discretion then residing in the home.
DISCLAIMER: This answer does not constitute legal advice nor does it form an attorney-client relationship between the questioner and responder. You should seek competent legal counsel in your jurisdiction to address any specific legal concerns you have.
If you are having questions such as the one in your post, you likely should be hiring an attorney to handle the case for you. In an unlawful detainer proceeding, both the tenant and the landlord must follow statutory procedures. The party who fails to follow statutory procedures likely will lose.
In WA, each adult is entitled to notice. Since the husband and the wife are not one person, they get more than one set of notice. If you serve only one set of summons and complaint, your complaint will be dismissed and you likely will be ordered to pay attorney's fees for the tenants.