Both of these historians study language, but is the one that studies a word's origin, as in where it came from.

The study of the origin of words (and of the historical changes in meaning) is called etymology so we can say:

Both of these historians study language, but an etymologist  is the one that studies a word's origin, as in where it came from

The other scientist could be for example a historical linguist, who studies how language systems change over the years.