OK I got the date wrong as I am old enough to think I can rely on my memory of what I was told 30-odd years ago. It was earlier than I thought as I can quote from (p244 of) Frank Vann's biography of Willy Messerschmitt which also shows that they shared an office.
In the autumn of 1969 he was working together with Kurt Tank in an office in the Arabellahaus headquarters of Messerschmitt-Bölkow-Blohm in Munich. Colleagues said what a pleasure it was to see the two great designers, who had for so long been rivals, working together on future projects.