Sony A7R II has a 42.0MP Full frame (35.9 x 24 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 8MP more than Sony A7R 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.
Another difference between these two cameras is that Sony A7R II's sensor lacks anti-alias (Low-Pass) filter. 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 A7R II and A1 sensor size comparison.
Sony A7R II and Sony A1 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Sony A7R II has 17% larger pixel area compared to Sony A1, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.