How does hash table search scales with number of keys?
In principle, they're similar in running time and space complexity.

In practice, the first one might have some modest advantages: possibly half as many accesses, less space overhead if some of the "secondary hashtables" were small (e.g., for a particular value of the first key, there aren't many possible second keys).

I would not expect to see a tremendous difference between the two, nor a difference in asymptotic worst-case running time.  Both have $O(1)$ expected running time for all major hash table operations, under some reasonable assumptions.