Both Sony A7R II and Sony A99 II have 42.0 MP resolution sensors but Sony A7R II's sensor is Full frame (35.9 x 24 mm ) and Sony A99 II's sensor is Full frame (35.9 x 24 mm ).
Since Sony A99 II's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Sony A7R II.
Another similarity between these two cameras is that both Sony A99 II and Sony A7R II 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 A7R II and A99 II sensor size comparison.
Sony A7R II and Sony A99 II sensors have exactly the same size so none of them has any advantage over other in this area.