Since I am good friends with the Bujinkan around these parts, I have had discussions about it.
It seems they may have met once...at a budo demonstration, but that's it. O Sensei never trained under Takamatsu.
When talking with Stan Pranin he reiterated that they "may have met" but that's about it.
From what I've gathered, Terry Dobson was actually a friend and student of Hatsumi Masaaki, but, their friendship ended when Terry decided to study Aikido solely, while rejecting a high position in Hatsumi's burdgeoning ninja movement.