Doesn’t the fact that it’s always the first chip that gets disabled disprove that theory? If it was to improve yield you’d see the other one being disabled at least some of the time.
It would be reasonable to assume that if both engines work, then the second is always the one to be disabled. Therefore to have the second enabled you'd need to find one where the first engine has failed, and has no other chip killing faults. Depending on the yield TSMC gets, these could be quite rare, so you'd have to have quite a large survey to find them.
Or as other people have noted, it could be an errata meaning the second core is broken, as this isn't the only possible reason.