I read the rules (though I don't own the game yet, Il'get it next week) and the only thing unclear to me was this : Do I have to lock more than 1 green die if two cards in play have a green lock ?…I thought you simply lock 1 and when you unlock it it gets locked again by the other card…but I'm not sure…

You lock one die per card with a die lock symbol.

Thank you. Seems pretty harsh. Poor investigators…

Truthfully, in my game play experience, multiple green dice locks don't seem to happen a lot. But I also always make it a mission to hit dice locks as soon as possible. The only things that I might prioritize more would be midnight effects, and that depends on what time it is.