Sadly, the two were already pretty well drifted apart by this point in the continuity. Hell, Seishin thinks in the anime on their growing distance when thinking of Tokujirou and his dad's relationship. They had to hurry their split because so much time early on was focused on the expanded Natsuno plot, that's my guess. The anime.manga basically had the "split" point be whether or not to dig up Setsuko's grave, with the nail in the coffin being Kyouko. In the novels, Seishin's actually in on more than one grave digging before their differences, present as early as the Ishida repression incident but manageable, start really snowballing to the point of now return.