This is actually pretty easy, you can just list multiple servers here in the LDAP configuration, separated by commas.
libldap takes care of the rest.
Edit: For later versions of FreeRADIUS 3 the LDAP module expects multiple instances of the server
config item. For example:
server = 'ldap1.example.org'
server = 'ldap2.example.org'
server = 'ldap3.example.org'
This is so internally, when the module is rewriting the URIs, they're already broken out into separate strings, which makes the code easier. It also looks a bit cleaner.