I've been trying to find a definitive answer to that for a while. A few others have been also. See this thread on 1-15 eight ball and look at the last post also asking about what happens if the 1 or 15 balls are sunk on the break shot.
There are no officially written rules which include that scenario in one-fifteen.
One guy posted that if you pocket the 1 or the 15 ball on the break, you win the game. But of course it is anecdotal, and not official:
I used to pay it all the time in a bar in the bay area. It's called one fifteen. The one and the fifteen have to go into the side pocket. You can't have the same side pocket. After that is played just like 8 ball except you put the 8 ball in the last pocket your last ball went into. You can't have the same last pocket. It's kinda like playing chess on the table. Your always trying to set up your side pocket shot without giving your opponent there shot in the side. When you rack the 1 and the 15 are he hind the 8 ball. Whatever sink on the break that's what you are. If you put the 1 or the 15 in on the break you win. That's it, really fun. You get lots of bank practice because it's last pocket.
Another Billiards Forum user, HL Fraser, mentioned another option in his comments on the one-fifteen rules page:
When we play 1-15 8 ball, the rule is that if either the 1-ball or 15-ball is pocketed on the break, it stays down as legally pocketed.
I've heard others say that if this happens you lose the game, but it is less common this way. I see more references to an automatic winning of the game.
Lots of people call one-fifteen 8 ball "Alabama 8 ball" as well. Searching for that might turn up some info.
The whole "15 in the left pocket, 1 in the right pocket" originated from the BCA rule book on or before 1948, but as part of the BCA 8 ball rules. Those rules are very brief and there is nothing about pocketing the 1 ball and/or the 15 ball on the break. The excerpt from the 1948 BCA 8 ball rules:
The rules of Eight Ball specify that the player pocketing the high numerical group of balls must pocket the 15-ball in the left side pocket, that is, in the side pocket to his left as he stands at the head of the table facing the foot of the table.
The player scoring the low numbered balls must pocket the 1-ball in the right side pocket. If the I- and 15-balls are not placed in these pockets as required by the rules, they are spotted and respotted until the player is successful in accomplishing this purpose.