Need to mention, that it is possible to use UV1 as source index by unchecking the “Generate Lightmap UVs” at merge time. It needs an extra step by opening the new mesh in the Static Mesh Editor and use the “Generate Lightmap UVs” there, using UV1 as both, source and destination.
Wouldn’t it be a useful feature to be able to select the source UV index for “Generate Lightmap UVs” with the Actor Merge tool?
For example, having a model, UV0 for a tiling material, UV1 for Lightmap layout, created in blender manually to have better control on the seams.
The current implementation forces UV0 as source. This should acutally not be a problem, because the Static Mesh editor already has this functionality, maybe just reuse code…
Another option coming into my mind: “Use existing Lightmap Coordinate Index” - actually, the merge actor tool is aware of the correct lightmap channel, because this is already set in the static mesh Light Map Coordinate Index. Using this information could even make it possible to merge models with lightmap on different UV channels.