The rules behind it
- Going to the Nether, the game searches up to 16 blocks horizontally around your converted position.
- Going to the Overworld, it searches up to 128 blocks horizontally.
- Both searches cover the full height of the dimension.
- The closest portal wins, measured in a straight line including height. A portal 5 blocks away sideways but 20 blocks higher is about 20.6 blocks away, so it loses to one 15 blocks away at your height.
- If no portal is found, the game builds a new one close to the converted position.
Why two portals end up sharing one
Nether distances are eight times shorter. Two Overworld portals 64 blocks apart are only 8 blocks apart in the Nether, well inside the 16-block search, so both can lead to the same Nether portal. Coming back, that Nether portal can only lead to one of them.
For separate links, build each Nether portal precisely at its own converted coordinates. At zero distance it is always the closest match, in both directions.
Three ways to fix a bad link
- Build at the exact coordinates. Convert the Overworld portal’s position with the Nether Portal Calculator and build the Nether portal there. At zero distance, no other portal can be closer.
- Match the height. Put the Nether portal at the same Y as the Overworld one when the terrain allows. A big height difference adds to the distance and lets another portal win.
- Break the portal that steals the link. A portal without its purple portal blocks is no longer a destination. Break one obsidian block of the frame to switch it off, and relight it later if you need it.
After fixing, walk through in both directions. A link that works one way can still pick a different portal on the way back.
Bedrock Edition
Bedrock searches up to 128 blocks in both directions, so shared Nether portals are even more common there. The same fixes apply.