You can configure an LDAP profile for each domain and users can authenticate against either domain. See also the setting ' LDAP ⇨ LDAP Settings ⇨ Global ⇨ LDAP Duplicate Mode'.
Once authenticated, a user can access the helpdesk module, and if the setting ' Modules ⇨ Authenticated ⇨ People Search ⇨ People Search Profiles ⇨ [profile] ⇨ Use Proxy LDAP Account' then helpdesk operators can set passwords for any LDAP profile. Keep in mind your bypassing LDAP/Domain user security with this setting enabled and relying entirely on PWM security model, as the password reset occurs using the PWM LDAP Proxy User account.