From the back cover:
At last - a thrilling TWO-PLAYER Fighting Fantasy Adventure!
YOU are Lothar, Warlock-Prince of the golden city of Gundobad. Your partner plays your twin brother and rival, Clovis, the Warrior-Prince. The time has come for one of you to succeed to the throne of Gundobad, and you must both face the Trial of Kingship.
But beware! The Trial of Kingship is not for the faint-hearted! Many dangers, traps and vile monsters lie in wait for the unwary, but with your sword and your spells, you may win through.
The other part can be found at Clash of the Princes: The Warrior's Way.