Apple - Toggle language by pressing Shift + Command

With freshest version (11.4.0 at 13.12.2017) of Karabiner Elements and Cmd + e for previous language source:

You can put this json gist to ~/.config/karabiner/assets/complex_modifications/<filename>.json, add gist rule in Karabiner Elements Preferences -> Complex Modifications -> Rules -> Add rule and Enable Languages -> Command + Shift to Command + e.

This will remap Cmd + Shift to Cmd + e and you can change language source by Cmd + Shift.

With macOS' system shortcuts you can't set keyboards shortcuts solely using modifier keys.

You need to specify an additional (non-modifier) key for macOS to accept and save your shortcut.

Sierra does have an extra setting (in system prefs/keyboard/input sources) for switching between certain keyboards via Caps Lock alone, but that does not help solve your problem.