I would double check that the US iPhone supports the bands (particularly mmWave) that the EU uses. I seem to recall something about mmWave bands being incompatible, and honestly other than that there isn't really any point to getting the US version instead.
My guess for having 2 mmWave antennas in the specified locations is 2 possible use cases, because of the nature of mmWave signals being easily blocked. Either it's in your hand and the side antenna is blocked but the rear one is open, or its sitting on a table or something and the rear antenna is blocked but the side one is open. Maybe?
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