Check out john@kucharik.com.
Kucharic has at least 3 or 4 different tandem themed jerseys, some with matching riding shorts and jackets Coming in all sizes, including kids. Even has a tandem cycling cap and do-rag!
Also check out Recumbent and Tandem Rider Magazine website. They have a neat tandem and recumbent themed jersey and jacket.
Hope this helps!
Rudy and Kay/Zona tandem