The PiTFT screen can run in three modes:
- With the HDMI turned off
- With the HDMI mirroring the screen
- With the HDMI as a separate display /dev/fb1
You currently have the mirroring setup so you need to create a bespoke screen configuration in config.txt to set the HDMI screen to 320x240 ASSUMING you physical display can handle this resolution - doubtful as it's not a 'classic' monitor standard.
To do this, you need to be familiar with the details on the Raspberry Pi site here in the section entitled Custom mode. Note the section above This one (Which values are valid for my monitor?) will help you find out if the HDMI screen will support this.
Make sure you have SSH enabled and working - it's possible to end up with NO display that you can type instructions on when playing with bespoke settings!
Assuming it does you can put the following lines in the relevant section of you config.txt file:
hdmi_cvt=800 480 60
The 60 value on the first line is for a 60Hz monitor and may need to be changed to match your equipment.
If this does not work (blank screen on the monitor) look at adding an aspect ratio to the first line or trying the overscan options (disable or scale) in config.txt
- I do not expect this to work at all due to the low resolution required.
- I would reinstall the software configured for two separate frame buffers giving you much more flexibility and control.