Aman,
If we are just talking synchronization then yes, you can synchronize from two separate AD environments. However, you must ensure that the versions are the same and that the user attribute you are using is guaranteed to be unique in both AD forests.
Authentication, on the other hand, is a little different. You specify the LDAP auth agreement to a single AD forest. So, unless you are talking about two different AD trees in the same forest AND you have the appropriate trust relationships in place, you are not going to be able to support LDAP authentication.
One of the things you could look at is MS ADAM. I have researched this but have not used it. The idea is that ADAM can pull info from separate AD envorinments and become a clearing house or aggregation point, if you will. That is probably a poor explanation but the characterization isn't really all that far off. You still have to ensure that your UID attribute is unique but ADAM can do some normalization (from what I understand).
The best place to start reading up on this is the UC SRND (www.cisco.com/go/srnd)
HTH.
-Bill (http://ucguerrilla.com)
