The instructions say to use the 5/8" plywood directly over the joists. If the t&g is sturdy enough and there is no deflection you should be okay. Read the directions thoroughly to choose what fits your situation best... I'm not sure what "car decking" is..new term to me. I would use screws that do not penetrate the decking in your situation.
Here's a link to installation instructions: http://www.holdenhumphrey.com/jh/hardibackerinstall.htm