Well, my partner and I use UB + Sed specifically because we think its the strongest, so uh, I'm not surprised to hear you say this, but I'm not sure exactly what to tell you. I wouldn't use Rook if I were you, hes weak for 2v2s. If you could convince your friend to use a different DG, Oak + LE is the other great 2v2 combo, in my experience. Its not a direct counter but it'll get you close enough to try to win by outplaying them. 

Uhh.

1. Get more grunts/minions asap as Sedna and Spit UB are gimped vs them.
2. Focus on Sedna or have enough burst to lay waste to UB.
3. It is enough to interrupt ONE of Sedna's heals to change the outcome of combat. Wait until she heals, count to around 6sec and then anticipate the next heal.
4. QoT + UB combo will be a lot stronger than Sedna+UB in early-mid game.

The problems with your builds is that you have no burst fire. Towers+Minions are the opposite of burst fire and this is what is needed to take down Demigods who have support demigods as their healbots. You should move to stun/hammer slam for rook in level 6+ and Oak should stack more auto attack damage instead of focusing on spirits doing all his dirty work. Get in there.

latly lost a game playing as sedna+ub vs oak/tb... they timed their shattert and pentience so well it was hard to heal once

but mainly they outdmg us so we had no chance even with healingwind monks 3 + heal lvl 3 to outtake the dmg the tb made on focus target < we played on crubible so it wasnt my map of choice. but very effectiv conter to healing