Both appear in Radio Days, though neither one is first-billed.
Manhattan
Radio Days
Diane Keaton appeared in these films written and/or directed by Allen:Play It Again, Sam (okay, Allen only starred in this one)SleeperLove and DeathAnnie HallInteriorsManhattanRadio DaysManhattan Murder MysterySimilarly, Mia Farrow appeared in these:A Midsummer's Night Sex ComedyZeligBroadway Danny RoseThe Purple Rose of CairoHannah and Her SistersSeptemberRadio DaysAnother WomanNew York StoriesCrimes and MisdemeanorsAliceShadows and FogHusbands and WivesLooks to me like Mia "wins" easily.
Yes, in the years before Annie Hall (1977).
yes
Diane Keaton turned 70 in 2016. She has never married, but had relationships with Woody Allen, Al Pacino and others. She adopted two children: daughter Dexter (adopted 1996) and son Duke (2001).
Zelig
Ronan Seamus Farrow
Ronan Seamus Farrow
The cast of Up Close - 1978 includes: Woody Allen as himself Diane Keaton as herself David Sheehan as Himself - Host John Travolta as himself
Dianne Wiest is a two-time Academy Award winner. She won Best Supporting Actress for two Woody Allen movies: Hannah and Her Sisters (1986) and Bullets Over Broadway (1994).
Ivan Kalmar has written: 'The Trotskys, Freuds and Woody Allens'