You need to convert "projectid" property as multivalue property,So that it can accomodate more then one project id.I see you have already added <multiple>true</multiple> to make it multivalue.
So I think missing piece is java code.
For a multivalue property you need to create list of values and set it into property.
Map <QName,Serializable> aspectValuesMap = new HashMap <QName,Serializable>();
QName QNameProjectId = QName.createQName(MYCO_NAMESPACE,"ProjectId");
List pids=new ArrayList();
pids.add(10);
pids.add(20);
aspectValuesMap .put(QNameProjectId , pids);
something like this.* Note this is just roughly typed code make sure to correct it syntactically.