@realeyez you do it by compositing over the same image texture. Just select each mesh in turn and keep the same image selected and bake one after the other. Each bake will be added to the same map. Not perfect, but it does the job.
On 2.79 you have to delete half of the mesh yourself (easily done by selecting a center vertex then using the Select -> Side of Active menu), but on 2.80 the Mirror modifier now has a Bisect option that should ignore one of the halves for you.
Snow : yes, but this would not select "implicit" hard edges (= the ones coming from the global Auto smooth angle). My goal was to be able to select *both* implicit and explicit hard edges in one operations, and RN's script does exactly that :)