It's a pretty easy to get this behaivor using ubuntu - I just enable keyborard layout switching by Caps Lock and Shift+Caps Lock automatically become Caps Lock switcher. When using elementary I try this:
gsettings set org.pantheon.desktop.gala.keybindings switch-input-source "['Caps_Lock']"
and then Caps Lock both enables Caps Lock and switches keyboard layout.
I prefer to swap Ctrl and Caps Lock. Keyboard -> Options -> Ctrl key position: Swap Ctrl and Caps Lock. In order to use Ctrl + \ to switch between keyboard layouts — Keyboard -> Shortcuts -> Universal Access: Next Keyboard Layout (Ctrl + \). So wen you press Caps Lock + \ layouts will change, when you press Ctrl(left) text input mode switches to upper/lower case.
