If you have different directories with different users, then you need 2 separate directory assistance documents. Put multiple servers in the Hostname field when you have more than one server serving the SAME directory.
Bob is exactly right. DA documents should have a 1-to-1 corrospondance to LDAP servers, except in the case where two (or more) LDAP servers have a replica of a single directory. Then, and only then, should you combine their DNS names and/or IP addresses into one DA doc and separate them with commas.
The comma separated names provides a type of “failover” for LDAP directories.