A global plugin is NOT a plugin on the Local Store. Many plugins have a disclaimer that they should not be deployed to an Adapter, such as UPNBind. To define a Global Plugin you need to navigate to the Advanced Tab in ODSM and it is the second section on the left side.
If you are doing a non-join combine of two domains into one, do not use a common top level OU. Define each domain to a unique OU in a common DC and then use the DC as the search base in anything searching for your users.
