When you step into a portal on either side, the game converts its X and Z coordinates as needed, then it looks for an active portal within 128 blocks of that location, and takes you to nearest portal found. If none are found within 128 blocks, it creates new portal. What this means?
Suppose you build your initial overworld portal at 0,0 and game creates a portal in the nether at 0,0. In the nether, you walk just 17 blocks over to 17,0 and build a portal there. Coordinates are multiplied by 8, so 17,0 in the nether translates to 136,0 in overworld. And guess what? You initial portal at 0,0 is more than 128 blocks away, so you will end up with a new portal in overworld.
Maximum distance you can have between portals in the nether and still have them go to same overworld portal, is about 32 blocks. (i.e. overworld portal at 0,0 and portals in the nether at 16,0 and -16,0)
By comparison, in the overworld you can have portals more than 2000 blocks from each other and still take you to the same portal in the nether.
It is much more useful to your purposes to build a tunnel between your two portals. The nether being 8 times smaller makes it far faster for travel. Perhaps the new portal will aid you in finding a map structure or a rare biome etc.
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The best puzzles are the ones we still don't know the answer to after all's said and done.