Qantas for example, when following the ILS the crew are under instrument or low vis procedures meaning that one guy is heads up and the other must be heads down and monitoring
I recall reading a 1970's accident report on a Caravelle (?) that crashed just short of the runway and into the water while approaching to land at Hong Kong Kai Tak airport Runway 31. It was raining very heavily. The captain was flying the approach and instructed his experienced first officer to stay heads down until he (the captain) was confident he could see the runway through the rain affected windscreen. This was not the "monitored" approach as we know it now. The captain was aware of refraction that heavy rain could cause. On the previous leg which may have been into Sung-Shan, Taipei, there had also been very heavy rain and the captain again directed the F/O to remain heads down while the captain landed.
Now this story is from my memory going back in the 1970s so bear with me. At Taipei and landing in heavy rain the captain was scanning as one does between instruments and outside to pick up visual cues early, when he noticed out of the corner of his eye that the F/O was gazing outside instead of being heads down. After landing he discussed this with the F/O and stressed the importance of (in this case) the F/O staying heads down and not to be tempted to glance up either out of curiosity or a survival mode. The point being that if both pilots are peering through a rain affected windscreen at the same time, the approach could become un-stabilised without either pilot being immediately aware of it because they were looking for the runway.
During the approach to Runway 31 Kai Tak, the captain was heads up and also rapidly scanning between flight instruments and a quick glance outside while the F/O was heads down. Around 1000 ft on short final the captain again noticed the F/O looking outside instead of disciplining himself to stay on the clocks. The captain immediately directed the F/O to stay heads down. When the captain saw the runway lights through the heavy rain he concentrated on maintaining a visual approach path but failed to arrest a high sink rate that was occurring and the aircraft flew into the water. At the inquiry the F/O admitted he was also looking at the runway on short final and therefore had missed the increasing sink rate.
He explained that it was all very well to have a SOP directing the PNF to stay heads down until the PF called visual. But that when getting closer to the runway there was an overpowering temptation to see what was going on outside. He succumbed to that temptation and shortly after with both pilots looking outside, they missed the sink rate.
When one pilot is flying on a low visibility rain affected approach it is difficult for him to continually ensure his PNF is staying heads down especially in a wide cockpit. An SOP is all very well, but human nature switches to survival mode and of course there will a compulsion for the heads down pilot to sneak a look up through the windscreen at the last minute on short final. This is especially so in heavy rain with partial visibility through the windscreen and when the phenomenon of rain refraction is present that often gives a false visual horizon.