Sorry for the delay in returning your email. I am just that far behind. For that reason also I am just going to give you a short answer.
Being an heir of God is to be an heir of eternal life. To be a joint heir with Christ is to share in His inheritance. All who are a member of the Body of Christ will share in Jesus Christ’s heavenly inheritance as the Kingdom Saints will in His earthly inheritance since He is “heir of all things” (Hebrews 1:2).