PTC has not tested this in any way and there is currently no ongoing plans or efforts to do so, so there is no guarantees that it would ever work. But, if you're looking to go ahead this, the following is included in the basic requirements:
-The properties wt.cognos.model.name and wt.cognos.naming.prefix must be unique on each of the Windchill installations that uses the shared Cognos installation. There are more details about them in the <Windchill>/codebase/properties.html file.
-The LDAP configuration in Cognos will need to be set up so that it can connect to all LDAP servers involved (including all WindchillDS instances).
The setup may work if these criteria are fulfilled, but there may be other configuration steps necessary. Again, this is an unsupported and untested setup.