Clash of the Princes were two books, released as a boxed set in 1986, that could be played as standard Fighting Fantasygamebooks or could be combined for a two-player experience. In the two-player game, two scores (action and status) are kept track of on a piece of paper in order to keep both players' game experiences synchronized. They were the first Fighting Fantasy books to drop out of print as retailers at the time were not fans of boxed sets.
... it was rather frustrating and extremely time-consuming to write and took a lot of work to get the mechanics sorted out (although the finished product probably doesn't show that). A friend suggested it one day after we'd been playing Ace of Aces and I foolishly took it up. The books were impaled on an unfortunate dilemma: were they single player books or a two player game? Martin [Allen] and I eventually came to the design decision that we would make the principal focus that each would be a single player Gamebook that had the advantage of being able to be played with its twin. We envisaged that they would be released separately, although simultaneously. However, Penguin chose to box them together and put the emphasis on the two player element. In addition, because of their experimental nature, there was a previously absent tension with Penguin which blew up and kind of soured our relationship a bit. Penguin also broke their contract with Martin and me by not putting our names on the covers.
At last — a thrilling TWO-PLAYER, two-book Fighting Fantasy adventure!*
The perilous Trial of Kingship awaits you! In the golden city of Gundobad, you are twin princes - one Warrior-Prince and one a Warlock-Prince - each with your own particular skills and strengths. It is time for one of you to succeed to the throne, and you must face the Trial of Kingship. But only one of you can win through. Which brother will it be?
Beware! Vile monsters and deadly dangers lie in wait. Two dice, a pencil, an eraser - and a friend! - are all you need.
Clash of the Princes features two new attributes that affect the gameplay of the books. Theses are action and status, both of which start at 0. action is a reflection of the choices made by the players during the course of the adventure with the actions of one player affecting the experiences of the other. status meanwhile is an indicator of the state of the player (dead, alive, wounded, imprisoned etc.). Should one of the players die in the course of a two-player adventure status should be set permanently at 1 for the surviving player. If the books are being played solo then the player's scores for both these attributes start at 1 and remain there for the duration of the game.
Unlike other Fighting Fantasy gamebooks, these attributes aren't recorded on the "Adventure Sheet", but on a piece of paper shared between the two players.