It's 256 blocks (128 distance - the 'cube' it looks within is 256 ont he side, with you in the centre) regardless of whether you're in the nether or not.
i.e.:
Create a gate in the real world, game scans for a gate in the nether within a 128 block radius of derived target position (real world x/8, real world z/8) in nether.
Create a gate in the nether, game scans for a gate in the real world within a 128 block radius of derived target position (nether x * 8, nether z * 8) in real world.
So yeah, if your places in the real world are more than 128256 apart, you'll need two separate nether gates. But you could position those two nether gates as close as possible in the nether. I mean if your places are just over 128256 apart, you could have two gates in the nether virtually right next to each other.
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