After setting up dual screen in Catalyst, it works fine while logged in. Then if I restart, in the GNOME login screen it is still working fine. During login there's some flickering, and now dual screen is disabled! This is 100% reproducible with the Catalyst version from about a week ago.
[The lack of detail is because I won't be back at my desktop for a few days. Just hoping this is a known problem with a known solution.]
See also this question