R.Rose solve this problem doing what they call 'controlled units', which means that the part of the serialization file corresponding to a folder (or package) is written in another file, and referenced in the parent file. This way you can edit independently and simultaneously various parts of the model.
Would be fine if umbrello implements a thing like that. 

At the very least, UMLDoc::getUniqueID() should generate a
random value instead of using an up-counter - else there are
ID conflicts if the submodel is developed independently of the
main model.

Here's my implementation proposal:
Each folder can have an optional "external_file"  tag attached to it.
The external_file value is a file name. The file location would be the same
as the main model file. A relative path may be given, which would also be
relative to the main model file dir.
The external_file directive acts as an "include" statement in programming
languages. I.e. the external file is opened and read into the model as
though its text appeared in the main model itself. (This means that the
external file is not itself a valid standalone XMI file, only an XMI snippet.)

CVS commit by okellogg: 

Implement saving of folders as separate files.
Externalized folders may contain UML objects or diagrams (but no further folders.)
For now, the folder files must reside in the same directory as the main model file.
Note that marking a folder for externalization does not save the folder to file;
that is done upon the next File->Save.
FEATURE:87252


  M +14 -2     ChangeLog   1.72
  M +29 -12    umbrello/listpopupmenu.cpp   1.67
  M +8 -0      umbrello/listpopupmenu.h   1.38
  M +87 -17    umbrello/umldoc.cpp   1.245
  M +13 -3     umbrello/umldoc.h   1.96
  M +71 -15    umbrello/umllistview.cpp   1.153
  M +2 -1      umbrello/umllistview.h   1.56
  M +101 -14   umbrello/umllistviewitem.cpp   1.70
  M +15 -1     umbrello/umllistviewitem.h   1.19
  M +24 -0     umbrello/umlobject.cpp   1.72
  M +6 -0      umbrello/umlobject.h   1.52
  M +25 -1     umbrello/umlview.cpp   1.213
  M +5 -0      umbrello/umlview.h   1.73

*** Bug 108585 has been marked as a duplicate of this bug. ***

SVN commit 575645 by okellogg:

New class UMLFolder gives folders a representation in the document.
Change the document model to match the listview appearance:
UMLDoc::m_objects is replaced by m_root, an array of modelviews with one
element for each of the Logical, UseCase, Component, Deployment, and
EntityRelationship models.
This prepares for implementing the loading of Rose controlled units.
Stuff known to need work:
- Support for external folders is temporarily broken
- On loading previous XMI files, the contents of user created folders
  appear okay in the listview but are not located inside the proper
  UMLFolder in the document
- The Datatypes folder appears multiple times in the list view
- Diagrams should appear in the XMI.extension of the respective UMLFolder
CCBUG:87252
CCBUG:81364


 M  +2 -0      Makefile.am  
 M  +0 -4      artifact.h  
 M  +9 -5      association.cpp  
 M  +6 -5      classifier.cpp  
 M  +4 -6      clipboard/umldrag.cpp  
 M  +0 -1      codeimport/import_utils.cpp  
 M  +4 -6      dialogs/umloperationdialog.cpp  
 M  +4 -3      entity.cpp  
 M  +0 -5      enum.h  
 A             folder.cpp   [License: GPL (v2+)]
 A             folder.h   [License: GPL (v2+)]
 M  +9 -1      listpopupmenu.cpp  
 M  +7 -0      main.cpp  
 M  +2 -1      messagewidget.cpp  
 M  +149 -15   model_utils.cpp  
 M  +28 -4     model_utils.h  
 M  +47 -9     object_factory.cpp  
 M  +2 -6      object_factory.h  
 M  +6 -11     operation.cpp  
 M  +63 -32    package.cpp  
 M  +10 -11    package.h  
 M  +3 -1      uml.cpp  
 M  +230 -222  umldoc.cpp  
 M  +32 -20    umldoc.h  
 M  +676 -753  umllistview.cpp  
 M  +75 -107   umllistview.h  
 M  +67 -166   umllistviewitem.cpp  
 M  +3 -11     umllistviewitem.h  
 M  +63 -7     umlnamespace.h  
 M  +14 -46    umlobject.cpp  
 M  +1 -7      umlobject.h  
 M  +53 -54    umlview.cpp  
 M  +2 -1      umlwidget.cpp  
 A             uniqueid.cpp   [License: no copyright]
 A             uniqueid.h   [License: GPL (v2+)]
 M  +0 -3      usecase.h  

SVN commit 580517 by okellogg:

Now that folders are first class document citizens, we can support external
 folders more solidly and fully:
UMLListViewItem::saveToXMI(): Move saving of external folders to UMLFolder.
UMLDoc::loadFolderFile(): Move to UMLFolder.
UMLFolder::{save,saveContents}: New.
This closes out the last remaining TODO mentioned in commit 575645, "Support
 for external folders is temporarily broken".
CCBUG:87252


 M  +1 -0      ChangeLog  
 M  +129 -66   umbrello/folder.cpp  
 M  +35 -0     umbrello/folder.h  
 M  +4 -76     umbrello/umldoc.cpp  
 M  +1 -11     umbrello/umldoc.h  
 M  +45 -11    umbrello/umllistview.cpp  
 M  +1 -2      umbrello/umllistview.h  
 M  +10 -56    umbrello/umllistviewitem.cpp  
 M  +1 -2      umbrello/umllistviewitem.h