Once viewing a (any) diff, to view files side-by-side or inline, there is the Compare: Toggle Inline View command that does just that, as @rio was on to above. I wanted to clarify what the command is called (and a comment does not allow images, nor snippets, hence this answer).

So, if you get a diff in inline view and want to view it side-by-side, just hit that command, here in the vs code command palette (ctrl-shift-p):

Command name for keybindings.json is toggle.diff.renderSideBySide:

{
    "key": "ctrl+alt+f12",
    "command": "toggle.diff.renderSideBySide",
    "when": "textCompareEditorVisible"
}

Hope this helps someone that came looking for this!