At the moment it is not possible to create ServiceReferences for specific targets of references with multiplicity="1..n".
See the mailing list for details: http://email@example.com/msg03024.html
A quote from Simon Laws' answer:
"The OASIS TC has proposed a solution to this issue (http://www.osoa.org/jira/browse/JAVA-9) but this isn't part of the API we have implemented as we have taken the V1 API. Can you raise a JIRA for this as I can't see one already and at least in that way we can track it."