Create a new flat world.
Build 2 Nether portals close to each other, Portal A and Portal B.
Make sure A is activated and B is deactivated.
Create a mechanism to toggle the portals, such that when a button is pressed, after sufficient delay to allow a person to use the previously activated portal before it is turned off, the portal that is currently on will turn off, and the portal which had been off will turn on.

If you hit the button and run through the portal in single player, when you attempt to return from the nether, you will come out at the location of the portal you originally used, even though it is no longer active. In my original test case, this placed me inside of a wall which I had to destroy to be able to move.

If however, you have multiple players, as long as one player remains in the overworld in proximity to the nether portals, when you come back to the overworld you will instead come out the portal which is currently active, which is the desired behavior which is not seen when playing single player.

This must be because in single player, when the only player is in the nether, overworld processes cease to execute, and thus the mechanism for swapping the portals does not actually happen until you go back to the overworld.

Solution would be to let certain types of processes continue instead of suspending them when the world changes.

If needed I can supply test world, video, images, etc.