Meshes Are Turning Black - Too Many Faces/Vertices?

I think I’m hitting some kind of threshold. These each start as .STL files from a intra-oral scanner (for your teeth) and processed through a proprietary program to make a virtual dental mold. Vertices: 41,301 and Faces: 82,542

I take the .STL and import into MeshLab 2016.12 To reduce the counts, I run Filter: Remeshing, Simplification and Reconstruction: Simplification: Quadric Edge Collapse Decimation. .5 Percentage reduction, Quality threshold 1. Vertices: 20,665 and Faces: 41,270 & save as .STL

Import into .STL into Blender 2.8; Add modifier: Decimate: Ratio .5 Vertices: 10,347 and Faces: 20,634 Export as FBX, Geometry: Smooth Face

Import into UE4. Static Mesh Editor: LOD Settings: Small Prop. General Settings: Light Map Resolution: change to 512. Assign M_Basic_Wall material.

If I bring either upper or lower mesh into it, Bake lighting, it looks fine although lower poly than I would like. But when I add one more, both go all black as in the image.

Any help or insights would be much appreciated.

Thanks! --Todd