Provided definition of composite service in lines 2315-2317 includes 2 not-needed conditions: " a single interface and described via a single service description" .
Any composite service may have as many interfaces as needed (to participate is as many aggregations or compositions as needed).
Any composite service may have as many versions of Service Description as needed to participate in as many aggregations or compositions as needed (at end of the day, it is a quite normal service).
Regards,
- Michael Poulin
P.S. Probably, the 'single interface' was meant as Facade-like interfaces that shield consumers from the interfaces of engaged services.