SAS;
I spent most of my early career two crew, there was a natural tendency to turn slightly towards your colleague when everything was relaxed. It was brought home to me by a very senior training captain when he pointed out that I had just flown 80% of a solid IFR trip looking at his instruments. I had never noticed that habit forming.
A few weeks later I had an FD failure on my side which took some time to notice because my attention was on the other side. Now I fly a mix of single and multi crew, and age has set in, but I very much agree with you, while the AI has the primary function, if you don't have speed and altitude nailed the AI can be letting you down very subtly and steering you to a whole can of worms.
SND