Welcome to 389ds.
Several ideas came to mind:
[1] If you can set up replication between the two LDAP server instances then the data will be available on both instances.
[2] If server2 suffix is different from server1 suffix, then you could use chaining.
(so that request to Server1 get forwarded to request2)
[3] using the Pass Through Authentication plugin (In that case only the bind requests will be forwarded. But that may not be enough depending how exactly the application is checking the ldap authentication)
Otherwise yo could always export the data from the other ldap server and import it on server1 (but you get in trouble if the data get changed ... )