Sounds like you are using automatic weights for only one mesh. You could select your second mesh, shift click the armature and use Ctrl+P to use automatic weight again. Alternatively I believe there are tools in blender to project weights from one mesh to another.
In any case this is probably not going to give you workable results. Automatic weight painting is a good starting point but you will need to make a lot of manual updates to both of your meshes. Depending on how different your second mesh is from your main mesh. It might deform very differently.