We're using mod_authnz_ldap and mod_authn_alias in Apache 2.2.9 (as shipped in Debian 5.0, 2.2.9-10+lenny7) to authenticate against multiple Active Directory domains for hosting a Subversion repository. Our current configuration is:

# Turn up logging
LogLevel debug

# Define authentication providers
<AuthnProviderAlias ldap alpha>
  AuthLDAPBindDN "CN=Subversion,OU=Service Accounts,O=Alpha"
  AuthLDAPBindPassword [[REDACTED]]
  AuthLDAPURL ldap://dc01.alpha:3268/?sAMAccountName?sub?
</AuthnProviderAlias>

<AuthnProviderAlias ldap beta>
  AuthLDAPBindDN "CN=LDAPAuth,OU=Service Accounts,O=Beta"
  AuthLDAPBindPassword [[REDACTED]]
  AuthLDAPURL ldap://ldap.beta:3268/?sAMAccountName?sub?
</AuthnProviderAlias>

# Subversion Repository
<Location /svn>
  DAV svn
  SVNPath /opt/svn/repo
  AuthName "Subversion"
  AuthType Basic
  AuthBasicProvider alpha beta
  AuthzLDAPAuthoritative off
  AuthzSVNAccessFile /opt/svn/authz
  require valid-user
</Location>

We're encountering issues with users that have accounts in both Alpha and Beta, especially when their accounts in Alpha are expired (but still present; company policy is that the accounts live on for at a minimum of 1 year). For example, when the user x (which has en expired account in Alpha, and a valid account in Beta), the Apache error log reports the following:

[Tue May 11 13:42:07 2010] [debug] mod_authnz_ldap.c(377): [client 10.1.1.104] [14817] auth_ldap authenticate: using URL ldap://dc01.alpha:3268/?sAMAccountName?sub?
[Tue May 11 13:42:08 2010] [warn] [client 10.1.1.104] [14817] auth_ldap authenticate: user x authentication failed; URI /svn/ [ldap_simple_bind_s() to check user credentials failed][Invalid credentials]
[Tue May 11 13:42:08 2010] [error] [client 10.1.1.104] user x: authentication failure for "/svn/": Password Mismatch
[Tue May 11 13:42:08 2010] [debug] mod_deflate.c(615): [client 10.1.1.104] Zlib: Compressed 527 to 359 : URL /svn/

Attempting to authenticate as a non-existant user (nobodycool) results in the correct behavior of querying both LDAP servers:

[Tue May 11 13:42:40 2010] [debug] mod_authnz_ldap.c(377): [client 10.1.1.104] [14815] auth_ldap authenticate: using URL ldap://dc01.alpha:3268/?sAMAccountName?sub?
[Tue May 11 13:42:40 2010] [warn] [client 10.1.1.104] [14815] auth_ldap authenticate: user nobodycool authentication failed; URI /svn/ [User not found][No such object]
[Tue May 11 13:42:40 2010] [debug] mod_authnz_ldap.c(377): [client 10.1.1.104] [14815] auth_ldap authenticate: using URL ldap://ldap.beta:3268/?sAMAccountName?sub?
[Tue May 11 13:42:44 2010] [warn] [client 10.1.1.104] [14815] auth_ldap authenticate: user nobodycool authentication failed; URI /svn/ [User not found][No such object]
[Tue May 11 13:42:44 2010] [error] [client 10.1.1.104] user nobodycool not found: /svn/
[Tue May 11 13:42:44 2010] [debug] mod_deflate.c(615): [client 10.1.1.104] Zlib: Compressed 527 to 359 : URL /svn/

How do I configure Apache to correctly query Beta if it encounters an expired account in Alpha?