ذَٰلِكَ عَٰلِمُ ٱلْغَيْبِ وَٱلشَّهَٰدَةِ ٱلْعَزِيزُ ٱلرَّحِيمُ
ذَٰلِكَ — the far demonstrative. Not "this" but "that". As was recorded in al-Bayyina, in Arabic the far demonstrative conveys taʿẓīm (exaltation) here: what is far is pointed to in order to raise the rank of what is being said.
ٱلْغَيْبِ وَٱلشَّهَٰدَة — the unknown and the seen. The same construct occurred inside a prayer in al-Zumar 39/46 (ʿālima'l-ghaybi wa'l-shahāda). In both places the pair is built in order to give the compass of the knowledge: two ends are named and what lies between them is taken in.
In Fuṣṣilat 41/2 it was tabulated that the names of the One who sends down change from sūra to sūra: there, in place of al-ʿAzīz · al-Ḥakīm, came al-Raḥmān · al-Raḥīm. Here there is a third combination: al-ʿAzīz and al-Raḥīm side by side.
I record this as my own reading; its ground is what is missing in the second verse: in the second verse the One who sends down was named not by a pair of names but by rabbi'l-ʿālamīn. The pair of attributes has been deferred by four verses and comes in the sixth — and comes as a pair that joins power with mercy. The same pair appears in Sūrat al-Rūm as well (wa huwa'l-ʿazīzu'l-raḥīm — al-Rūm 30/5); it will be mentioned in al-Rūm.