I would use Continuous AF (AF-C) mode with a Single AF point. a1 AF-C Priority Selection - Focus a5 AF Activation - AF-ON Only and use the AF-ON button to focus.
Which version of the 300mm f/2.8 are you using? Which 2X TC are you using? How are you supporting (tripod and head)your camera and lens? What shutter speed are you using? Are you stopping the aperture down at least one or two stops when using the TC? Good Luck and Enjoy your Nikons!