The butternut and heartnut will probably cross-pollenate one another - but you never know for sure until you see them bloom and determine whether or not they have compatible/complementary pollen-shed/nutlet-receptivity patterns.
I have one hearnut that's so late vegetating/blooming that nothing else I have will pollenize it(unless maybe the black walnuts might).
The black walnut will probably benefit from one or more other black walnuts in the area, as they tend not to bloom in concert with butternuts or most heartnuts.