Both are independent certifications. You can write one without having another. But SCWCD is considered to be more useful in day to day working life. SCDJWS is little advanced technology and will be little harder to understand.
See also SCWCD or SCDJWS - Seeking Advice. However the servlet related and web application deployment objectives can come in handy when dealing with Web Services. Whether it is actually worth also putting up with JSPs and tags to get the SCWCD must be your personal call. SCWCD is easier than SCDJWS.