I don't get why 5970's double fixed tessellation units should have to do with anything. It's not like one tess. unit of one GPU can help the other. They both help the GPUs they are in. Performance scaling shouldn't be different when there's Tessellation than when there's no tessellation.

Saying the second tess. unit should help in tess. heavy environments is like saying the additional 1gb of RAM in 5970 should help in 2560 + 8xAA.