This is an un-documented situation and is usually to be clarified before starting the game.
However, the most common way people play last pocket 8 ball is that you can not have the same last pocket as your opponent. If your opponent ends up with the same last pocket as you, you get to choose which pocket will be their last pocket.
Here are four different folks confirming this from various other 8-ball forums:
A few folks confirming this to be a "generally known" standard part of the last-pocket eight ball rules:
One rule that we also liked was that both players could not have the same last pocket.
That is a standard Last Pocket rule, in my experience. If you take the same last pocket as the first established player, said player gets to pick your last pocket.
And two more players confirming it in a different thread:
In Puerto Rico, if you pocket the last ball in your opponent's pocket, your opponent can nominate your pocket, including side pockets.
I've always played that the opponent gets to pick the shooter's last pocket (if the shooter makes their last ball in the opponent's last pocket)...even choosing the side pocket. I've also seen it played where the shooter's pocket, in this circumstance, became the opposite pocket (on the other end of the table).