This paper concerns whether the simple mixing rule for thermopower, theta = Sigma t(k) theta(k) works for a heterogeneous, two-phase mixture, each with different types of charge carriers (k), where t(k) and theta(k) are the transference number and thermopower for the kth-type carriers, respectively. We examined the thermopower of p-NiO/n-ZnO two-phase mixtures with an artificial series or parallel configuration and a real polycrystalline configuration as a function of the mixture composition, temperature, and microstructure. It was found that the mixing rule is generally valid for mixtures with the minority phase along grain boundaries. The anomalous, repetitive transitions from n-to-p-to-n-type with increasing temperature are attributed alternations of the predominant conduction paths with temperature. This mixing rule is compared with conventional theories of the effective thermopower of mixtures.