P-Type Metal Oxide Semiconductors (MOS) are promising sensor array elements, as their surface properties and charge transport mechanism differ from those of their ubiquitous n-Type counterparts. As such, the magnitude and temporal evolution of their response can provide supplemental, independent information on the nature of target gases, thus allowing for improved discrimination in electronic nose systems. Moreover, the formation of a heterojunction between n-and p-Type MOS can be leveraged to enhance or tune sensing performance. Here, we report on the processing and characterization of stacked SnO2/NiO thin films of varying thickness. Bilayer heterojunction films are deposited via magnetron sputtering, with or without an intermediate thermal anneal. We show that the addition of a p-Type NiO layer dramatically alters the behavior of the n-Type SnO2 underlayer, reversing the response pattern to Volatile Organic Compounds (VOCs) and causing a shift in the optimal operational temperature of the device. Plausible physical mechanisms leading to the emergence of such behavior are discussed.