It doesent refer to the two individual barrels that make up a set of DR barrels.

It means that one might have a 405 NE for example and a set of 375 flanged barrels that "match" the 450's in style finish and sighting arangment. Thus one action two sets of barrels. That match.

I dont know if anyboody has made a set of DR barrels with two differing rifled tubes, either in twist rate or rifleing profile. There would be no reason to do so apart from economy and given the cost of the rest it is insignificant. It might make no difference, but on the other hand it could make the adjustments for regulation interesting to say the least.