How many inches above the bar that comes welded from autopower do folks weld in the additional bar that comes with the cage?

Or, if you don't have autopower, how far apart are your door bars?

I don't remember seeing anything in the CCR about minimum seperation.
I put mine below the inside door handle, low enough with padding to still use the handle. It's perfect level to rest my elbow on when seated, and ok for sittng on when getting in and out.

There are no limits in the NASA CCR's. So it is up to you and whoever the cage builder/welder may be.

I like to place my 944 door bars 6" apart (measured from the bottom of the top bar to the top of the bottom bar), with two spreaders between them. My upper door bar is approx. 13" from the top of the bar to the rocker at the point where it meets the forward hoop. There's no science to these numbers, but for most drivers the overall height seems to allow good ingress/egress (especially with "Extended-Style" door bars) while enabling retention of the factory door intrusion beam.