I used the starter content's wall objects, an atmospheric fog, a directional light and this is the result:
As you can see, it's obvious where the new object starts.
Answering as of 4.21.1, one reason for problems with fluid lighting may be due to issues with UV maps of model. Check if:
In this image:
Lights are static 2cd intensity, 300 attenuation, enough to cover a single building. Materials are default material.
Thank you Mr. Gregory for reminding about overlapping UV case. So, in case of overlapping UV, we get jagged lighting like this.
Ideally, we dont want corners to be lit up. Most low poly textures have baked ambient lighting which is least costly. If you absolutely want dynamic lighting, use ambient occlusion and proper texture/normal maps to remove corner effects.
Should look like this:
More customization(for low poly lighting) available if you make texture not light using by custom greyscale map from 3d modelling software.