What unusual experience do these directors have in common?

What unusual directorial experience do John Boorman and Dario Argento have in common?

Hints: Ron Howard could do this, but hasn’t come close. And Francis Ford Coppola didn’t do it when he had the chance.

Answer: Boorman and Argento directed their daughters in sex scenes — Boorman in Excalibur, Argento in The Stendhal Syndrome.