Hi Dan,
sorry for the delay. I was on holiday last week and didn't see your message until today (the backlog was pretty big ).
I'm attaching a zip file to this post with a copy of a second LDAP auth plugin directory (current as of today for 1.9.9+). You need to unzip the file and put the 'ldap2' directory under .../moodle/auth, just like the regular LDAP plugin.
Once you put it there, you'll see a second LDAP plugin, called "LDAP server (2)". You need to configure it like the regular one, set the preferred one above the other in the order setting screen, and you are done.
As I said in my old message, make really sure you don't have users in both LDAP servers with the same username. Otherwise Moodle will think they are the same user and you will have all kind of troubles.
Saludos. Iñaki.