The joint memorial will take place in Los Angeles on Thursday, sources close to the family told ABC News. A public event is also in the works.
Fisher, 60, passed away last Tuesday, days after suffering cardiac arrest as she was traveling from London to Los Angeles. The next day, Reynolds, 84, died from a stroke.
Reynolds' son, Todd Fisher, told "Good Morning America" last week that before her death, she "said she missed her daughter and wanted to see her again."
"I think she wanted to be with her. I'm not joking when I say she left to be with her, and I'm happy about that. That's the only thing I'm happy about," he said.
Todd Fisher previously told ABC News that his sister's funeral would be a celebration and that her daughter, Billie Lourd, would be arranging the details.
"Carrie loves a good party, so, you know, I'm sure she'll want to throw a party for her," he said.
Lourd broke her silence Monday about losing her mother and grandmother in the same week.
"Receiving all of your prayers and kind words over the past week has given me strength during a time I thought strength could not exist," Lourd, 24, wrote on Instagram. "There are no words to express how much I will miss my Abadaba and my one and only Momby. Your love and support means the world to me."