How can I avoid this lighting problem when vertex snapping 2 walls (same model) and why is one wall brighter than the other?
1: use the ‘world normal’ viewmode (under buffer visualisation) to make sure that the walls have the same (and the correct) normals.
2: make sure you have a lightmass importance volume in your scene, this greatly improves the GI of lightmaps and often fixes this issue.
3: make sure you have the correct lightmap UVs, and that they have a high-enough resolution. use alt-0 to view the resolution of your lightmaps, and check under ‘build options’ for each of your static meshes (in their LOD properties) to make sure that the ‘min resolution’ and ‘lightmap coordinate index’ are correct.