Nunes

Alternatively to having an audio interface with more than one stereo out, you could create an aggregate device in your mac using your interface and your buit-in output. By doing so, you are able use both outputs separately in logic. Sorry for the system language, but the pic shows the pop up under audio midi setup. Sorry for the system language, but it's under audio midi setup in a Mac.

Files > Movie > Export Audio to Movie Do not check the box for the movie's audio and voilá