Yeah I think they are intended to be symetrical. Either that or Jo didn't realise til book four because she said that one was pivitol in the series in both the sense that it is the change from a non-coporeal Voldemort to a now corporeal Voldemort.
In terms of structure PS and DH are similar as they are both books with broken up adventures whereas the rest are all part of one linear plot whereas PS and DS zig zag towards the end.
Plus in PS and DH Harry rides on Sirius' motorcycle with Hagrid away from danger to safety. It is revealed at the end that Snape turns out to be saving Harry. Harry visits Gringotts in both the only two times he does it in the series. He also encounters a dragon in PS and DH.
In COS and HBP Harry puts his trust in a book he just finds lying around. He also visits Knockturn Alley in both and sees Draco and Borgin and Burkes in both. Also the students were almost sent home in both.
In POA and OOTP Harry sees Lupin, Sirius, the Knightbus. He also learns about the circumstances of his parents deaths in both. His patronus and the dementors appear in both.
Alot of parallels.
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