Doesn't really make sense to me that "cat <jpg>" should render the bitmap to the screen. 'cat' is a concatenation tool. 'cat'ing two JPEGs together isn't the same as 'cat'ing two text files. So why? Use another tool, don't break the idiom.
cat /etc/debian_version /etc/debian_version shows the file twice
Shockingly, cat sixel1.six sixel2.six sixel3.six or img2sixel file1.jpg file2.jpg file3.jpg works in the exact same way: cf https://github.com/csdvrx/sixel-testsuite/blob/master/3-sixe...
Simply screenshot and edit the result when you are done.
But if you want to create another tool, for example to trim white boarders and concatenate, by all means please do!
Follow the example of https://github.com/hackerb9/lsix who wrote a great tool!