print to JPEG file - optimal output sharpening

Just to clarify this is being done to prevent the need to create separate A2 and A4 collage layouts.

I totally agree with Jao. I tested my original suggestion of using the A2 print layout JPEG with resizing to A4 in the LrC Export module using the same sharpening setting. This results in an over-sharpened A4 image file!

What appears to work is to create a 2nd A2 print layout JPEG file with 'Print Sharpening' unchecked. Then resize it to A2 and select the same output sharpening in the LrC Export module. When I compared to an actual A4 size file created in the Print module the sharpening was virtually the same. Interestingly the  resized A2 file looks very slightly sharper than the Print module A4 sized output file. Not sure why.

You indeed absolutely want to do the output sharpening at the final printed size. The difference is very noticeable when you test (at least to me). Lightroom does the sharpening at the resolution you specify in the output and so the result is very different for a A2 print at 300 ppi as it is for a A4 print at 300 ppi. Even though the A4 file will be much smaller (about a factor of 4), the print from the A4 specific file where you had Lightroom scale and sharpen at the correct size will appear much sharper. 

Do note that the difference might not be observable to most people but I tested this double-blind with people asking what print they prefer and they generally prefer the prints that were sharpened at the correct size.

The simpleset solution is to create the A2 layout JPEG file with the desired output sharpening. Then Use the LrC Export module to resize it to A4 dimensions and apply the same desired Output Sharpening in the Output module. That way you only need to use one layout and stll can control the target sharpening. I doubt you will see a difference in sharpening when compared to A4 layout Output Sharpening in the Print module.

What is the purpose of custom file dimensions ?

If outputting to JPEG you should be entering your page size in the Custom File Dimensions. More Help here:

https://helpx.adobe.com/lightroom-classic/user-guide.html

The final, optimal sharpening is best done in original size and under visual control on the JPEG. Lightroom has sophisticated manual settings for it. Then you have to test the optimal settings by comparing the monitor and printout. After upscaling, there is definitely more loss of detail and sharpness than with reduced output.