We ended up needing a similar setup on our moodle site which we run v1.9.9 and I followed this guide to setup LDAP and Active Directory authentication:
http://moodle.org/mod/forum/discuss.php?d=74279
Basically all you are doing is creating a second LDAP plugin in moodle. There are quite a few people on this above post who seem to run into problems but I just did exactly what was being described and found it to work quite well. If you have any other questions on how to do this I'd be glad to help.
acjohnson [at] pcdomain [dot] com