Sony A6400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Sony A7S II has a 12.0MP Full frame (35.6 x 23.8 mm ) sized CMOS sensor and features Bionz X processor.
Sony A6400's sensor provides 12MP more than Sony A7S II's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the A6400 and A7S II sensor size comparison.
As seen above, Sony A7S II has a 2.3x Larger sensor area than Sony A6400. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Sony A6400 and Sony A7S II sensors have been tested by DxO and the results show that A7S II has a better overall score of 85, 2 points higher compared to A6400's score of 83.