have a look at the wiki page on
GAL_Sync_Account
I found it very helpful when i needed to add multiple external LDAP sources to my GAL.
You can only add one external source using the Admin Console,
but using the CLI, you can add more.
I have used this method recently to share GALs between 3 different
ZIMBRA setups.
so you can create a new source to add your .org domains
and then another source to add your .us domains
tomoki