Hi, i think the thing that’s making the surfaces look split-up is a break in your so-called phong shading.
Normally when two faces are attached, the surface gets a phong-smoothing to make the faces appear as one, however, two seperate object’s faces won’t get smoothed out with each-other (as they’re seperate meshes/objects).
First attempt could be to check import settings oncemore.
Try making sure:
Mesh Compression is disabled
Normals AND Tangents are imported, NOT calculated
(The phong smoothing is what unity refers to by the Smoothing Angle, recalculating the tangents would change the smoothing.)
“All normals” in terms of “all normals”, “face normals” or “vertex normals”?
The latter is the important thing here.
I am not familiar with the process in Maya, but in Max you can use the “Edit Normals” modifier (applied as a shared modifier to both meshes), select the vertices on the edges, in the “Average” section set a threshold (just to be sure) and hit “Selected”. This will align the vertex normals on the edges of both models to be absolutely the same, which should give you the desired result.
As I said - I am not familiar with the process in Maya though, but maybe that’ll give you a clue.
If that doesn’t help you may indeed run into some shading issues.
In Maya you select both meshes and than go to face mode this way both of the meshes go to face(poligon mode) and then you select the faces on both of the meshes. Than you go to mesh display and Average. This smooths faces across separate meshes and imports to Unity the same way.