Assuming all flight segments are NW-operated, I fail to see why NW couldn't create a new, separate reservation/PNR for the two travelers -- having yield management open up two seats in the original fare class, if necessary -- and substitute it for the erroneously split original reservation, which could subsequently be deleted. I'm sure this would require a tad bit of work on NW's part, but if its error created the problem in the first place, then NW should supply the solution.