If you are going to do both anyway do the SCWCD first - mainly because you'll probably wrap that one up in less time. However the SCWCD is certainly not necessary for the SCDJWS; the cross-over is minimal according to Val's SCDJWS Specifications-Objectives Matrix i.e.
SCDJWS Objectives: 8.3 Given a scenario, implement J2EE based web service web-tier and/or EJB-tier basic security mechanisms, such as mutual authentication, SSL, and access control. Servlet 2.4 specification: Chapter 12 (Security)
9.1 Describe the steps required to configure, package, and deploy J2EE Web services and service clients, including a description of the packaging formats, such as .ear, .war, .jar, deployment descriptor settings, the associated Web services Servlet 2.4 specification: Chapter 13.1.1 (Packaging and Deployment of JAX-RPC Components), 13.4 (Deployment Descriptor Diagram)
For the SCWCD all you need is the Head First Servlets and JSP: Passing the Sun Certified Web Component Developer Exam (SCWCD) (aka Head First SCWCD) and the specs: