The pair replace Spice Girl Melanie Brown, who announced her departure in July, and This Morning star Holly Willoughby, who quit in May.
The pair, who are close friends, have both previously appeared on the ITV quiz and challenge show.
They will debut alongside host Keith Lemon this autumn when the show returns with eight 45-minute episodes, plus two hour-long specials, on ITV2.
Actress and reality TV star Atack said: “Every time I’ve been on Juice in the past I’ve had such a good time, and now I’m doing it with two of my best mates and I’m a ruddy team captain!!! Can’t believe it.
“I dread to think what we’re going to be faced with, but Whitmore should know that I won’t be backing down and she is going to have some tough competition.
“But more importantly a wonderfully hilarious time. Bring it on!”
Love Island presenter Whitmore said: “I’m so thrilled to be part of the Celeb Juice family.
“I’ve been the biggest fan for years, I used to go and sit in the audience when I first moved to London and can’t believe I’m a team captain now!
“I can’t wait to see what Keith has got lined up for Emily and I this series… From past experiences, nothing would surprise me.
“I cannot wait to get stuck in and lead my team to victory each week… I love you Emily, but watch out! I’m so excited to start!”
Lemon, real name Leigh Francis, added: “Can’t wait to start new celeb juice with old friends. Exciting times ahead! The funnest job ever!”
Brown, known as Scary Spice, originally replaced Paddy McGuinness, who in turn replaced Fearne Cotton when she quit after 10 years to pursue other projects.
Show bosses previously suggested Brown would return as a guest in the near future.
Willoughby left her team captain role after 12 years on the show.
