Here is how The Cake Twins hanging out with Phineas and Ferb and Agent P assigned on a mission with Princess Yuna goes in Candace and Stacy's time with Sebeena.
Pound Cake: It's really nice of your parents to let us hang out with your parents to let us spend the day with you and Ferb Phineas. This is gonna be fun.
Phineas Flynn: I know, Pound Cake. I'm excited to have lots of fun with you and your twin sis.
Pumpkin Cake: Who wants Cinnamon Rolls!?
Ferb Fletcher: I would like one.
Pound Cake: I know that look, Phineas. What're you thinking about?
Phineas Flynn: How about we'll make our own Cinnamon Roll Bake Sale?
Pound Cake: Sounds good to me, What do you think, Pumpkin Cake?
Pumpkin Cake: My thoughts exactly, Pound Cake.
Isabella Garcia Shapiro: Hi, Guys. Whatcha doin?
Phineas Flynn: Oh, Hi, Isabella. We're planing our Cinnamon Roll Bake Sale. Would you guys like to help us?
Isabella Garcia Shapiro: Sure, Phineas.
Pound Cake: Would you like to help too, Buford. You can help Pumpkin Cake bake.
Buford Van Stomm: You bet.
Pumpkin Cake: Baljeet, How good are you as a salesman?
Baljeet Tjinder: Very well I guess. Count me in.
Phineas Flynn: Hey, Where's Perry?
Meanwhile with Perry.
Perry the Platypus/Agent P: (secretly enters his secret lair)
Princess Yuna: Hi, Parry.
Major Monogram: Afternoon, Agent P. We've recruited Princess Yuna, The Mario Bros and Sonic the Hedgehog on a very important mission. Carl, The view.
Carl: Right away, Sir. (shows the screen with Dr. Eggman and Bowser and his family)
Perry the Platypus/Agent P: (shocked with Bowser and Dr. Eggman together)
Princess Yuna: Oh no! Bowser and Dr. Eggman are on to something!
Major Monogram: You must help Princess Yuna save Equestria.
Carl: And somehow, Doofenshmirtz has summoned you all for your help.
Perry the Platypus/Agent P: (looked at Yuna about her efforts)
Princess Yuna: He maybe the only one who knew about Bowser and Dr. Eggman's whereabouts.
Major Monogram: The fate of Equestria and Danville are in your hands and hooves. Good luck, Agent P.
Carl: Same to you, Princess Yuna.