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Yes, one DLL can contain several add-ins; however, these add-ins must be written in the same language. To create such a DLL, you must ensure that DllRegisterServer registers all DSAddIn objects in the DLL so that each object has a different CLSID and ProgID.
If a DLL contains multiple add-ins, each one could manipulate objects in different object models. For example, one add-in could manipulate Developer Studio objects, and another could manipulate Visual Basic objects.