Creating two Ubers tiers (Henceforth referred to as Ubers and Ubers-B, such that Ubers-B excludes the titans of Ubers) would create a definitive error in Ubers's dynamic with OU. OU is intended to be as inclusive a metagame as possible, but it also strives to ensure that none of its members are uncompetitive or broken. Note that there is a distinction between those two descriptors; broken pokemon are simply overpowered and centralizing, but uncompetitive pokemon/strategies lower the overall level of skill necessary to succeed to the point where a much more skilled player has a decent chance of losing to a much less skilled player. Meanwhile, the Ubers tier focuses only on banning uncompetitive pokemon/strategies: Of the extremely paltry selection of entities banned from Ubers, a strong case can be made for all that they violate the skill-based integrity of the metagame.

If you break the tier in two, you come across an interesting conundrum: you are taking the total set of pokemon (minus those which are uncompetitive) and eliminating from play those which overcentralize the metagame with the goal of providing a more inclusive and balanced metagame. Sound familiar? That's because Ubers-B, at its heart, is just OU. At best, Ubers-B represents an OU-style tier above OU (similar to how OU and UU coexist, or UU and RU, or RU and NU).

Now, you may think that this tier could work, that it would just be an OU-style tier with more powerful pokemon. Here is where we hit the problem: You cannot create an OU-style metagame with Ubers. The Ubers tier is unique in that its threats are, by nature, centralizing. They are literally the best that gamefreak has to offer, and many of them (one specific example being the box legendaries) are created by gamefreak to be ridiculous nukes or unbreakable walls. Every single member of the Ubers tier is a centralizing threat, some more than others, and that is a fact that Uberers just deal with. The tier thrives on. But creating a sub-tier with any of these threats that has the option of banning them if they are broken will assuredly fail. You're introducing the weaker Ubers, the ones that had to get tested to leave OU, back into OU. Surely you can see the problem here. There's a reason that they left.