OWA Login Authentication (Choose which one?)

Hi All,

I am facing an issue with the OWA login page.

From External login in I will require to login twice (First at TMG 2010, Second will be OWA 2013 interface)

From Internal login I will require to login once only (OWA 2013 interface)

I understand that I would need to put the same Authentication so External would only type in password once.

I tried putting both to basic, External manage to work as in user only require to login once, but the Internal no longer showing OWA interface, only the authentication box for username and password. Management mention that it is require to having the interface on Internal login as well.

My Aim:

1. External only login once, with TMG interface / OWA interface
2. Internal with OWA interface

Currently I am putting the authentication as TMG (Basic), CAS (Domain\Username)

Any ideas where can I start to check?

Thanks and Regards,

Low