Open vim with each file in a new vertically-split window from the CLI

To open 'file1' and 'file2' in vim with file1 on the left half & file2 on the right half of the 'window', use '-O'

mark@home:~$ vi -O file1 file2

If you're new to the split-mode editor, you can use ctrl-w, left-arrow and ctrl-w, right-arrow (and up and down) to navigate between the windows.