Pentax K-S2 has a 20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor. On the other hand, Fujifilm X-T1 IR has a 16.0MP APS-C (23.6 x 15.6 mm ) sized CMOS sensor and features EXR Processor II processor.
Another similarity between these two cameras is that both Fujifilm X-T1 IR and Pentax K-S2 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the K-S2 and X-T1 IR sensor size comparison.
Pentax K-S2 and Fujifilm X-T1 IR have almost the same sensor size so none of them has any significant advantage over other in this area.