Actaully - I have it figured out! The trick (at least in my situation) was to completely ignore the setup instructions for the 5" display. With a fresh install of Buster for my Pi4B (2020-02-13-raspbian-buster-full), and my 7" display hooked to HDMI0, and 5" Seeed display hooked to HDMI1, I checked ‘Can see black boarders around screen’ on the initial config, completed the initial Pi settings, then rebooted. Then simply used the GUI ‘Preferences->Screen Configuration’ and selected the 5" display, and selected ‘Orientation Right’, applied and rebooted. Screen came up fine. BTW - do not attempt to change resolution in GUI for 5" to 640x480 - for some reason this changed something and even after trying to revert back to 720x1280, the 5" stayed black - even after reboot. Had to do another fresh image to get over that test!
Ideas on why the modifications to config.txt would not work? If not - no big deal as I currently have it running on my Pi4B with no config.txt modifications - just used the GUI settings on the Buster desktop.
Thanks,
Joe