I get the impression that keeping separate was more of a concern when Mr. Hill started out as a writer, out of concern that he would be dismissed as riding his father's coattails or that his own work wouldn't be taken on its own merit. He had success with HSB and 20th CG before he was 'outed', so to speak--once that happened, what would be the point of denial? Better to enjoy each others' work, IMHO, and if they share ideas, etc., who's to blame them?
Their styles are similar, but not identical--it's pretty easy to pick out who wrote what bits in
In The Tall Grass--so I don't think either is afraid of being confused with the other at this point.