Sony A9 II has a 24.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features BIONZ X processor. On the other hand, Sony A1 has a 50.0MP Full frame (35.9 x 24 mm ) sized BSI-CMOS sensor and features Dual Bionz XR processor.
Sony A1's sensor provides 26MP more than Sony A9 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 A9 II and A1 sensor size comparison.
Sony A9 II and Sony A1 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.