UV2 Lightmap Atlas Creation Outside of Unity?

I know how to make a second uv channel, what is spinning my brain is how to set up UV2 to work with the lightmap atlas in Unity, where there are 1-?? 1024 images applied to the entire scene. How do you lay out the UVs evenly like that outside of Unity?

All you have to do is to create UV2 for each object separately and Unity will do the packing into maps during Lightmap baking process, i.e. each object has to be fit into [(0,0)(1,1)] square (if you have more than one mesh in the file then you have to unwrap them separately) and Unity will simply set offset and scale for each of them during baking.

You want to use one lightmap for multiple objects then u will have to merge them as one single mesh and unwrap it. or else there is commercial plugin for 3D Max called Flatiron it bakes the lightmap inside 3d max and you can use it inside unity with legacy lightmapped shader group. Check their intro videos you ll get a rough idea how the pipeline works.