Here is Decision 27/11 in full, which explains the principle when a provisional ball has been played;
A player entitled to play a provisional ball from the tee plays it into the same area as his original ball. The balls have identical markings and the player cannot distinguish between them. The following are various situations and the solutions, which are based on equity (Rule 1-4), when the above circumstances exist and one or both of the balls are found within a search of five minutes:
Situation 1: One ball is found in a water hazard and the other ball is not found.
Solution 1: The ball that was found must be presumed to be the provisional ball.
Situation 2: Both balls are found in a water hazard.
Solution 2: As the player’s original ball is lost in the water hazard due to his inability to identify it (see analogous Decision 27/10), the player must proceed under Rule 26-1 with respect to the original ball (estimating the spot where the ball last crossed the margin of the hazard, if necessary – see Decision 26-1/17); his next stroke would be his third.
Situation 3: One ball is found in bounds and the other ball is lost or is found out of bounds.
Solution 3: The ball in bounds must be presumed to be the provisional ball.
Situation 4: Both balls are found in bounds, whether in a playable or an unplayable lie, and (1) one ball is in a water hazard and the other is not or (two) both balls lie through the green or in a bunker.
Solution 4: One could argue that both balls are lost. However, it would be inequitable to require the player to return to the tee, playing five, when the player has found both balls but does not know which is the original and which the provisional. Accordingly, the player must select one of the balls, treat it as his provisional ball and abandon the other.
Remember that the principle in Situation Four does not apply to the question at the start of this article, because no provisional ball was involved.