The splitting of the comments over different blocks in doxygen is not possible. You will get messages like:
.../aa.h:5: warning: ignoring \startuml command because PLANTUML_JAR_PATH is not set
.../aa.h:5: warning: startuml section ended without end marker
This functionality is by default available in doxygen. You don't need to use (in this case the startuml / enduml commands. Just have a look at the HAVE_DOT, UML_... and ...GRAPH...` settings.