Nikon D810A has a
36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features EXPEED 4 processor. On the other hand, Canon 6D MII has a
26.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features DIGIC 7 processor.
Nikon D810A's sensor provides 10MP more than Canon 6D MII'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 D810A and 6D MII sensor size comparison.
Nikon D810A and Canon 6D MII have the same 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 Canon 6D MII has 39% larger pixel area (33.19µm2 vs 23.83µm2) compared to Nikon D810A, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.