Select your grandfather, then under where you put his details you'll see a list of options. One of them says "Add partner/ex" select that. You didn't ask, but heads up, if you set the second wife as your grandfather's primary partner, his late wife will be set as one of his ex-partners rather than an extra partner. If you want to set the late wife as an extra partner, you'll need to select "More actions..." then "Change partners". I'm sure you can figure it out from there.
