i think the brahmashtras have been already decided irrespective of the voting- the mentors together have selected few contestants and those will be brahmashtra round participants. They will all perform in the same way as the chakraviuh round contestants and the judges probably will decide the two who will go through to the final or agnipariksha.
I think it will be janata voting that will decide who from the bramhashtra round will go to the agnipariksha round but I am not sure. I recall at the Chakravyuh round Shaan said that this is the final time judges will be giving their verdicts and voting will decide the fate of the contestants after this round.