2 tenants only 1 signed AST

Hi.
Im about to serve a section 8 notice on my tenants who are a married couple but have just realised that when I took deposit and signed the AST only 1 tenant was present so I only have 1 signature on the AST, although the agreement is in both the couples names. I didnt get round to get the other tenant to sign.
Is this going to be an issue or should I only serve the notice to the person who did sign the AST?
Any advice appreciated.