Sed + Oak is very good Cata 2v2
Sed + TB is competitive because of how they cover eachothers' flaws
Ereb + Oak
Rook + Oak or Ereb
But, Sedna is basically neutered by a Rook, so can't use her against him.
If you notice, Oak is always one of the best demigods in 2v2. This is because he is a General, and he is weak early game, strong end game. 2v2 doesn't have a lot of brutal combat until the end-game which makes him an "optimal" choice. He also has minions which are quite strong in 2v2 games.
Beast can obviously be used with any general, pretty much a plug-and-play.
Rook is pretty damn strong 2v2 cata but at the same time ... raped by minions.
But, Oak + Ereb or Oak + Beast are the two most versatile demigod doubles that work together well, have few weakness and few really bad matchups.