There have been problems with sharing the main (back-buffer) depth buffers, see this thread:
http://www.ogre3d.org/forums/viewtopic.php?f=4&t=54096&p=368313#p368313
We will probably revert this feature until the new depth sharing design is complete :
http://www.ogre3d.org/forums/viewtopic.php?f=4&t=53534
It is only a mild memory/performance setback in some extreme rendering pipelines, so...