How to combine two images into one on a Mac?
I often have to do this with images of plots of data.  I use the command line tools that come in the Imagemagick package; I think I installed it on my system with MacPorts. You could also choose to install with brew (brew install imagemagick).
The actual tool you want to use from Imagemagick is the convert tool.  If you have your two 320x428 images, say a.png and b.png, you can do
convert +append a.png b.png c.png

to create a new file, c.png, that has the a.png on the left and b.png on the right.  Alternatively, you append them vertically with -append (instead of +) and a.png will be on top of b.png.  With convert, you can do a ton of other things.  For example, you can switch to a different image format for the output
convert +append a.png b.jpg c.tif

This isn't a GUI application, but maybe some others might have a better solution.  Alternatively, you could put this in some sort of automator script.
2020-12-10：
I used it on 2020-12-10 and now the correct code is
convert +append a.png b.jpg +append c.tif