A report may be known through only one route yet receive the expression hasan sahih. One explanation is hesitation or variation in critical assessment: some critics place its narrator at the hasan threshold while others judge him at the sahih threshold.
The expression can then convey a sense comparable to "hasan or sahih", with the disjunctive element omitted. This is a contextual explanation, not a universal grammatical rule to impose automatically on every use of the phrase.
Practical conclusion: When encountering a composite label, examine the available transmission routes, the narrator's actual precision, and the particular critic's usage.