Sorry for the year later bump, but I had trouble when reimporting after adding a 2nd UV channel too. While experimenting I unchecked ‘Generate Lightmap UVs’ in the LOD0 and Import Settings dropdowns, saved the asset, and then reimported. Then I rechecked generate lightmaps and now I have both of my channels, plus the 3rd generated by UE4 for the lightmaps. Hope that helps someone.