You can go either 2 routes that I know of, maybe someone else can chime in;
1. Take the auth/ldap plugin and make a copy, rename the folder, update all the appropriate files and fields in the folder with the new plugin name. Install the newly copied plugin and enable it, configure for the 2nd AD forest.
2. You can setup an OpenLDAP Meta Backend that reads from both AD Forests and have Moodle authenticate to the Meta backend. You must have unique users across both forests, everyone must have different CN's, sAMAccount, etc so that Moodle knows who is who.
Jason