Probably a little bit late, but it might help others.
No, there's no need for another toolbar.
When creating a toolbar the bitmap is loaded to a CMFCToolBar internal object, which can be reset by CleanUpLockedImages. After this another bitmap can be loaded with a call to LoadBitmap (same as during creating). AdjustLayout then does the re-paint job.
m_wndToolBar.CleanUpLockedImages();
m_wndToolBar.LoadBitmap(IDR_MAINFRAME_2, 0, 0, TRUE );
AdjustLayout();