Merge 2 armatures and 2 meshes then parent the armature to the mesh, without losing their weight paints? I have 2 arms i made separately and i copied it, but when i merge the meshes and bones and then reparent the bones to the mesh, one bone is weighing both arms, but the other isnt doing anything.
this is probably a naming issue : 


the armature will create vertex groups with the bones names 
when you join the meshes groups with the same name get merged
when you join armatures bones will get new names if other with the same name exist
so you end up with one armature controlling both meshes


to avoid this ensure that your armature bones have different names before parenting each mesh to the armature