open files in a split

vim -o index.js index.html

to open them vertically:

vim -O index.js index.html

You can pass a number if you want to only open a certain number of splits.

vim -O2 index.js util.js index.html will open the js files in vertical splits, and index.html as a buffer but won't make a split for it.

This is a thing that I seem to want to do at exactly the frequency where I look up how to do it one time and have forgotten it by the next.