I would dual at 9, because the main thing about beeing a breserker is the rage, other bonuses are like the fighter ones and its usualy smart to dual a fighter at level 9 rather then wait, especialy since you have other fighers in the group, you will want a cleric as soon as possibile.
If you're powergaming, go for 9th. But with the game so easy to beat, it adds more challenge to shoot for 13th.
To the Righteous belong the fruits of violent victory. The rest of us will have to settle for warm friends, warm lovers, and a wink from a quietly supportive universe.
Since you have a 4 person party, XP should be coming in rather well. If you only have SoA and dualled at 12, you'd probably get 13 in cleric in Ch. 6. I'd probably do it.
And I'd definately wait til 12 if I had ToB installed. You could even do WK if you wanted to add on the XP so you'd dual earlier.