Two-tone nibs are ordinary gold nibs partially plated with a white metal, usually platinum, palladium, or a member of the same group. Sheaffer in the '30s was the first to adopt this feature; Conklin briefly used it on its Nozacs and Parker on Duofolds, but these trials were quickly dropped, supposedly after threats of legal action by Sheaffer. In any event, two-tone Conklin and Duofold nibs are rare.
The role of Sheaffer in Parker's abandonment of the two-tone Duofold nib would seem to be overblown, however, since Parker almost immediately adopted a two-tone nib for its new flagship line, the Vacuum Filler/Vacumatic. In the early '40s Parker put a two-tone nib with a "V" motif on its new Vac-filling Duofolds, and the original Skyline came with a two-tone nib as well. The plating of Sheaffer's two-tone nibs was by far the best: few show significant plating loss today, whereas Parker, Conklin, and Eversharp nibs with pristine plating are scarce.