Just reviewed the B12 bylaws and public announcements around the vote approving Cincinnati, UH, BYU and UCF as new members.
Interestingly, UT and OU don't have any veto right over admission of any new B12 members. That is, even together, they don't have the votes to block admitting members. Which is unfortunate because, if they did, they might have been able to leverage it during the G5 raid (or any future P12 raid) to get a better exit deal.
Unless you're interested in the legal minutiae, feel free to stop reading at this point.
In this B12 statement, it was announced that admission of the G5 teams was "unanimously approved by the eight continuing members" under Section 1.5.2.b.3 of the B12 bylaws. That Section requires approval of a "Supermajority of Disinterested Directors," meaning 75% of the Board of Directors (on which each member has one rep).
Assuming all 10 member schools were considered "disinterested" with respect to the G5 admission vote, that means 8 votes required (i.e., no UT/OU vote needed). And, going forward, approving the approving admission of any additional members (e.g., P12 schools) would require 11/14 — again, no UT/OU vote needed.
So, the only scenario where I see the possibility of UT/OU leveraging their votes to get a better exit would one where the B12 wants to add additional member(s) and at least two of the Irate Eight schools have been poached by another conference (such that they would be willing to vote with UT/OU in vetoing the new members). Obviously an extremely unlikely scenario.
Finally, even if that scenario existed, it could be further complicated if the remaining B12 members took the position that UT/OU and their defector friends were "interested" in the admissions vote (basically, they have a unique interest that would cloud their objectivity in voting) such that they would be blocked from voting. I actually expect that this might have already happened to UT/OU in the G5 vote given the careful wording of the B12 statement mentioned above.