In the case of 757/767, they coordinated on general cockpit design and handling qualities, but little else.
True- what few realize is that the 757 was largely a Renton design team , and the 767 was largely a Everett design team. Yes there was some interplay on common items and a lot of control/backup/baseline coordination mainly based on what worked historically. The 737 was largely a Renton design team then 747 was largely an Everett design team. And there were some employee transfers between them.