All the evidence so far gathered fits both Dr  Grippen' s theory and Professor Heissmann' s  However, the predictions that these theories make about the result of the planned experiment cannot both be true
Ttherefore, the result of this experiment will confirm one of these theories at the expense of the other
David and Jane both think they know how to distinguish beech trees from elms, but when they look at trees together they often disagree. Ttherefore, at least one of them must have an erroneous method.