I would say a basic reaction to one another would be like "Oh, you're a ***. . ." with varying levels of mistrust/hostility depending on background of the individual. Basic negativity on the soft end of the scale, and a knee jerk reaction to attack on the harder end. I'd say it's still fairly common for one side or the other to "Take revenge" or something like that, but that's mostly a generational "Your kind/family killed my great great something, and my father, blah blah blah" giving a level of basic bad blood and specific reasoning for many characters, while over all it's mostly rumor and innuendo for those not directly linked(That they know of) to negative happenings between the two sides.

That way our two guys could get along, but still have the background of knowing ll about blood feuds that just compounds the issue of whatever happened in the past to make them hostile to one another. As for when the thing happened, I'd say so long ago that maybe only really, REALLY old fogies have any idea what it is, and for the most part, they ain't sharing. :P