You would also have to have a rule something like, both the cue ball and the object ball have to be placed at least one ball away from inside a pocket -- i.e. you could place an extra ball behind them, and the extra ball would fit cleanly, without falling in the pocket.
You could take turns doing this -- taking turns setting up the shot, and then you each shoot from there. I think the player setting up the shot should only shoot once (they would shoot second, since they set the shot up for their opponent), while the opponent should shoot first, and then also third -- to give them a chance to capitalize if they got out of the trap on the first shot. Kind of playing horse -- you get a letter if you scratch; you also get a letter whenever your opponent scores. You should also get a letter if you can't fit the extra ball between the pocket and the balls you placed, without touching the balls or the extra ball falling in the pocket -- i.e. you placed one of your balls too close to the pocket.
Actually, it would be more interesting if, after you start the game, each turn after that, you can pick up
either the object or the cue ball to start the next turn, but not both. That way, you would be forced to be creative with the balls in different positions. Of course, if either player scores a ball, then the next player in turn places both, to start again. If the object ball is pocketed in a neutral pocket, then it would go on the spot, and you place the cue ball where you want... After a scratch, the next player can place both (like the start of the game).
We could call this, "One Pocket Horse"
