Please have a look at this screen capture first:
The problem with combining UV(W) tags is that they typically hold all information. Yes, it doesn’t look that way. The not needed parts can be all in the corner of the map. I think it changed at one point from the left upper to left lower corner. These “dead” polygons are practically there, even they are useless. So, to even take the ASCII information and copy and paste would just double the amount of data. In the model you have shared, the UV(W) information is not sorted even, as in mesh.1 first, then mesh.2.
To sort this out, I split them for a moment (deleted then extra UV Tags) and then merged them again. Please test this, as each model is perhaps differently set up. To make sure it works, check the UV meshes of the split objects, if it works as they are at that moment, then the chances are high, that it will work when merged.
I hope that helps. If there is anything else, let me know, I’m happy to look into it.