Two 6970s should have a higher hashing rate than one 6990. I wouldn't necessarily return them.
Not really, if you OC the 6990.
You can get 410-430 mhash/s per core, about in line with what a single 6970 would get overclocked.
Specs wise a 6990
is two full 6970 cores as well (1536 shaders each).