Oct 29, 2012 10:37 PM|Angie xu - MSFT|LINK
I don't have the codes available about authenticate against LDAP with backup server, but you could try
Enterprise LDAP Fail-over Support,
LDAP server is such a critical resource, many large software enterprises will install a series of primary and secondary LDAP servers to make sure that the organization can continue to operate in the case of an unforeseen failure. Nexus Professional’s Enterprise
LDAP plugin now provides you with the ability to define multiple LDAP servers for authentication.
Hope it helpful.