I’ve had similar problems.
The reason this is happening is because of the size of your lightmaps. Check to see if you have objects that have very high lightmap resolution. I’m no expert on this but, the way the load is distributed is across the computers is, each object’s lightmap is computed by one thread of the PC. So if you have 100 objects that have say 64 lightmap resolution, and have one object that has 1536 lightmap resolution, those 100 objects would finish up very quickly on one PC and the object with 1536 resolution would go for 4h, on the other PC.
The way to fix this is to make all the objects have similar lightmap resolution. If you have a big object that requires a bigger lightmap res, then simply divide it into smaller objects and reduce the lightmap resolution.
I hope this helps.