for anyone who's interrested in this; the solution is to derive from CXTPTabClientWnd and add a sort function that removes all items and afterwards add them again in sorted order:

 

void CXRSortableTabClientWnd::Sort() {
 // Get the set of all opened MDI client windows
 std::set<CMDIChildWnd*> clients;
 xrApp->GetMDIClientMap(clients);
 // Insert the set into a multimap sorted by document title
 std::multimap<_tstring, CMDIChildWnd*> sorted;
 for (std::set<CMDIChildWnd*>::const_iterator i=clients.begin(); i!=clients.end(); ++i) {
  CString strItem=GetItemText(*i);
  sorted.insert(std::make_pair(_tstring(strItem), *i));
 }

 m_bLockReposition = TRUE;

 // Remove all current tab items
 m_pActiveWorkspace->DeleteAllItems();

 XTPWorkspaceNewTabPosition position=GetNewTabPositon();
 SetNewTabPosition(xtpWorkspaceNewTabRightMost);

 // Add tab items in a sorted manner
 for (std::multimap<_tstring, CMDIChildWnd*>::const_iterator i=sorted.begin();i!=sorted.end(); ++i) {
  CXTPTabManagerItem* pFoundItem = AddItem(i->second);
  pFoundItem->SetHandle(i->second->GetSafeHwnd());
 }

 m_bLockReposition = FALSE;

 SetNewTabPosition(position);
 Reposition();
}

does anybody know how to reorder CXTPTabClientWnd tabs by code?