Well, to be quite honest, RB will be a better offhander due to the attack for a really long time yet. 12.25/16 is a .766 ratio which is right around the cutoff for bagging RB with 1450+ attack and 40%+ haste, assuming relavent AAs and a superior mainhand. Without knowing your AAs nor attack level I would still have to presume that TS/RB will be your best combonation for some time yet. (I'd note again that 12+1 is not 13 but closer to 12.25 or 12.33 depending on the dataset you like.)
Even if you had no Ferocity and did have Ambi/Sinister Strikes with maxed worn attack, I'd be a little shocked to see RB/Whip ever outperform TS/RB. TS/Whip becomes preferred at ~1450 attack and with 40% haste, although it's still pretty marginal at that point.