No you cannot update Dynamo Core and Dynamo Revit manually at the same time, this is because the sandbox version is a stand alone version and does not include anything revit related(eg Sandbox is Dynamo Core). You still need the revit side of dynamo to be able to update Dynamo Revit.
You can update just the core side of dynamo as a manual process though this is highly advisable not to do because it will not contain anything related to Revit and it has not possibly not gone through any revit test.