From http://forum.openscenegraph.org/viewtopic.php?t=8890&view=next

CameraUpdateCallback.cpp

class CameraUpdateCallback : public osg::NodeCallback { 
private: 
    osg::ref_ptr<osg::Camera>           mOtherCam; 

    public:    
        CameraUpdateCallback(osg::Camera* otherCam) : mOtherCam(otherCam) {   } 

    osg::ref_ptr<osg::Camera>           m_sourceCamera; 
    void operator()(osg::Node* node, osg::NodeVisitor* nv) { 
            osg::Camera* cam = static_cast<osg::Camera*>( node ); 
            
            cam->setViewMatrix(mOtherCam->getViewMatrix() ); 
            cam->setProjectionMatrix(mOtherCam->getProjectionMatrix() ); 
            
            traverse(node , nv); 
     }      

... 
camera->setUpdateCallback( new CameraUpdateCallback( currentView->getCamera() ) );