I don't mold things So I'm not an authority figure or anthing, but I would go the old Hotglue and Elastic method. You mess up, heat her up and adjust. And you hold onto whole "mobility" Idea.
These are Adam and Spase's shoulder/bicep armor. Industrial strength velcro should home them together as Nintendude described. If you wanted to attach the two permanently together, I would recommend E-6000 glue (can be found at Hobby Shops like Hobby Lobby and Michaels).