How to Correctly Parent the Rifle and Robot together?

When I import the Rifle and the Arms(Model I made) to Unity Both work well separately when adding animator controller to each. but how can parent the Rifle to to Arms so that when I press R the Reload Animation for both the Rifle and the Arms play well Simultaneously?

Can you show what options shown in the picture or more am I Supposed to configure and how?

thanks in advance. jlampel 

I checked every single animation of the FBX files and they work great.