How to do a double grace note?

I have not figured out how to add grace notes to a chord. I have a voice in octaves and I would like to add a grace note to both upper and lower note. No matter what I select it always adds either just one grace notes or 2 grace notes, one after each other.
I attached a picture of a bar from Nocturne Op 6 by Chopin which shows what I want to do.
Thanks
Frank