How do I merge smoothing shaders seemlessly without joining objects? I'm having an issue with creating a game asset for the Creation Engine.

I am trying to have two objects, a head and body, match seamlessly together without joining them. They don't and it's because the normal vectors have a mind of their own, unless I join the objects and weld them together (alt+m).

The smoothing shaders look like a complete mismatch between the two objects. I've tried editing them both, by having two selected and entering edit mode, and then trying to calculate the normals based upon selection, but nothing I've tried seems to be working. Does anyone know how to fix this?
Remove the smoothing shader > merge those 2 objects using CTLR+J > smooth that one (merged) object > apply the modifier > once you're done and you're not going to edit the model anymore, in edit mode select one edge that between your head and body and press P.