I believe that actually merges the two columns as a single key, as opposed to using two keys (at least, thats the impression I get from the somewhat ambiguous man page). Thus, it would work fine if the two keys are next to each other (as they are in this case) and both strings. But for a numeric sort on column 1 and a string sort on column 2, you need to specify the keys independantly (-n -k 1 -k 2).