Two-color quantum well infrared photodetectors (QWIPs) with two stacks of QW series have been grown by molecular beam epitaxy and processed into mesa structure devices with only two ohmic contacts by photolithography and wet chemical etching. By changing QWIP parameters, including barrier height, well width, doping level and period number, the total bias voltage can be distributed to the two stacks in such a way that the stacked structure will show different photoresponse characteristics. The photocurrent spectrum measurements demonstrate that sample 1 can work alternately between the two atmospheric windows of 3—5 μm and 8—12 μm by tuning the voltage, while sample 2 can photorespond simultaneously to the irradiation of the two atmospheric windows. In this paper, the physics behind the two-contact type of QWIP is discussed. The voltage tunability and the simultaneous photoresponse are attributed to the change of photoconductive gain with the bias voltage and the distribution of the total bias between the two series. We here focus the discussion on the voltage tunability of sample 1. Compared with the three-contact-per-pixel structure, two-contact-per-pixel structure can greatly facilitate the dual-band focal plane array (FPA) device fabrication and increase the FPA fill factor.