table sharing across sites leads to session slaying as users jump from site to site

We are currently sharing tables out in array() via $db_prefix. This is our alternative to multisite as we have unique trees of Drupal core + mods + etc on disparate places on disk which are not necessarily persistent given our deployment tools. This allows tables for www.domain.tld (users*, etc) to be referenced by 'child' sites, such as foo.domain.tld or bar.domain.tld, which have the necessary bits stood up in their $db_prefix. We have observed that:

1.) Log into 'parent' site via CAS
2.) Log into 'child' site via CAS
3.) Observe that CAS-obtained session at 'parent' site is no more.
4.) Log into 'parent' site via CAS
5.) Observe that CAS-obtained session at 'child' site is no more.

The same behavior is observed whether you log into the 'child' first as step 1, etc.
We currently run 6.x-1.0 (!) where the bug was first observed. Upgrading to 6.x-2.3 did not seem to resolve the issue.

Our current corpus of tables being shared out for these child sites are as follows:

'actions'
'actions_aid'
'authmap'
'node_type'
'permission'
'profile_fields'
'profile_values'
'role'
'sequences'
'sessions'
'term_access'
'term_access_defaults'
'term_data'
'term_hierarchy'
'term_relation'
'term_synonym'
'trigger_assignments'
'users'
'users_roles'
'users_uid_seq'
'vocabulary'
'vocabulary_node_types'
'workflow_access'
'workflow_scheduled_transition'
'workflow_states'
'workflow_transitions'
'workflow_type_map'
'workflows'

This is kinda, sorta, maybe, possibly inline with observations on http://drupal.org/node/869926