Lightmap UV reimport

Hi, I’m actually working on a couple of trees that i need to redo using some layer technics, and since the meshes needed new uv i tried to reimport them into the engine but apparently no matter waht, even if i uncheck the box concerning the autogeneration of the uv… id does it aniway and since i m using a 3 UV it basically destry the second one… Is that a bug or is there an other solution cause i dont want to merge the new one with the original at this point because of the name of the meshes.

Thanks.
Jonathan