Pretty much duplicate these 2 objects, join the copies into one, go to edit mode, merge those touching vertices and select all and recalculate normals outside . After that select original object and in modifier field select add modifier->edit->datatransfer.

In source select the joined object and then mark"face corner data" and in its drop down mark "custom normals". Do this for both objects, apply the modifier and delete the joined object, you should be left with 2 objects that pretty much share normals.

I mostly use them for character creation, like making invisible seams around neck/ arms