Sinah and nawm are not simply two interchangeable synonyms. Drowsiness is lighter than full sleep, which makes their order intelligible.
Negation independently covers both: la ta’khudhuhu sinatun and wa-la nawm. The verse therefore denies both drowsiness and sleep of God.
Two dimensions must be distinguished:
1. Doctrinal statement: neither condition applies.
2. Rhetorical sequence: the lighter, earlier state is mentioned first and the fuller state later.
The grammar of negation and the appropriateness of the order reinforce one another.