Hello,
I don't know too much about directory services, but I think that there are two possible solutions for this problem:
1.- Synchronization between two sources. This way you'll have the information duplicated, just against the purpose of the directory services. And probably it will modify the flow or the proccess of the data manipulation.
2.- Metadirectory. A metadirectory is something like a general view of different directories (LDAP, AD, other directories or databases). I think it uses connectors between the main server (join engine) and the existent directories to ask for the info they contain and then generate a global LDAP view. That is, the different sources of information are still there, but you have a general view and it looks like if you only have a directory server.
I don't know how to implement these services, but maybe you can check Sun's documentation on metadirectory.
Possible tips you can search on the net are: ldap + active directory + synchronization, PAM + Active Directory, metadirectory, LDAP + AD connectors, ....
Bye,
jmiturbe