The thing is... Only one of the potential universes has any observers. Hypothetical observers have dont actually observe anything, which means that you cannot reason backwards from the number of observers each universe would in theory have.

If what the super-theory predicted was that both universes existed, but was agnostic about which bit of the multiverse you were in? Then you could make a guess based on the numbers (and a very large absolute number of observers would get it wrong). But that is not how this problem was posed. And given the way it was posed, at most one of the theories can be correct, and will be observed to be so by whichever percentage of its inhabitants get to that stage of understanding. The hypothetical inhabitants of the other universe have no bearing on the matter due to not existing.