Book 1
Title: PJO x KC: The Symbol Thief
Plot: Apophis is starting to wake up. He has just enough energy to send a part of his spirit to earth, where it makes its way down into Tartarus. In Tartarus, it converses with the spirit of Kronos and both come upon a plan to take over the world together. Together, the spirits of Apophis and Kronos make use of their minions in the mortal world, and steal the Pharaoh's Crook and Flail from Egypt and The symbols of the Big Three from The Greeks. They shift the blame onto the opposite side, causing the Greeks and Egyptians to find out about each other and go to war. The greeks then receive a prophecy, thrusting Percy, Annabeth, and Grover onto a quest to recover the objects lost by both sides to form a truce. On the other side, Carter and Sadie Kane receive visions from Horus and Isis telling them what they must do, and the two leave on their own journey (With Bast or Set or Bes or any combo of that) to recover the objects to prove that they hadn't stolen them.
Beginning: Kronos and Apophis are shown conversing and discussing their plans before they tell their minions to start carrying them out. After the objects are stolen, Percy will be shown at the camp, learning about the stolen symbols and receiving the prophecy. At the same time, Carter and Sadie will be shown receiving their dream visions.
Middle: At this point, both sides would have left for their quests while the others have started to make their way to the place where the battle would happen. Along the way, they end up meeting each other and realize that they have a common goal. They team up to find the symbols and create peace between both sides.
End: They recover the symbols, fighting off Kronos and Apophis' minions in the process, and give the symbols back to who they belong, thus proving their innocence and making sure that both sides become allies instead of enemies.
Future: This can be turned into a series, where the main villains are Kronos and Apophis. The storylines can be similar to the actual PJO books, which would mean you would need to make it a 5 part series, or similar to Kane Chronicles, which can be 3 part.
Book 2 idea: Book two can be similar to the sea of monsters, but instead Percy, Annabeth, Carter, and Sadie will have to cross the lake of fire in the duat and reach an island in the middle, which is where a part of the Sun God's soul is kept, due to the fact that it will be needed in the future battle once Ra is completely awakened (by the last book).
Book 3 Idea: This can be similar to the Titan's Curse. In this story, Set will turn rogue and will try to sabotage their missions and it will be up to Carter and Sadie to stop him. At the same time, Percy and Annabeth will be forced to go on a quest to defeat Atlas and save Artemis.
Book 4 Idea: The demigods discover an entrance to the Labyrinth within their camp, and also discover one which allows them to reach Brooklyn House from CHB within seconds. They realize that the enemies could use this to their advantage and go on a quest to recover Ariadne's string before the enemies, needing it in order to prevent Kronos and Apophis from maneuvering through the labyrinth. Similar to the 4th PJO book, there will be a fight, but the fight will happen in the labyrinth between the forces of Kronos and Apophis on one side and the Egyptians and Greeks on the other. Ra finally awakens at the end when it seems as though all hope is lost, and his arrival gives them new energy and they win the battle.
Book 5 Idea: The greek demigods and Egyptian magicians will make their final stand in New York where they will defeat the forces of Kronos, before moving their forces to ancient Egypt, where they will go to assist the gods, who are holding back Apophis until the demigods and magicians can arrive. Carter, Sadie, Annabeth, and Percy must ALL read an execration spell to defeat Apophis once and for all.