Here's a weird one: My grandparents are divorced but have lived together for almost 40 years post-divorce. My grandmother technically has her own apartment, but she has a bedroom at my grandfathers house and my parents say to just send one invite for both of them to his house. They do everything together (and we suspect they might be boning but we aren't asking because, you know, grandparents, gross), so how should I address this invitation?