How to assign Caps Lock (first lang) and Shift+Caps Lock (second lang) in Ubuntu 18.04?

Question

I want to change language layout via CapsLock for the first layout and Shift+CapsLock for the second layout. How can I do this?

For example, I have two input sources in Settings → Region & Language → Input Sources: English(US) and Russian

In gnome-tweaks → Keyboard & Mouse → Additional Layout Options → Switching to another layout: I checked option “Caps Lock to first layout; Shift+Caps Lock to last layout”

And then I checked in Settings → Region & Language → Options: it shows me that “Alternative switch to next source” equal “Caps Lock to first layout; Shift+Caps Lock to last layout” but it didn’t work

I tried reassign typing shortcuts in Settings → Devices → Keyboard → Keyboard Shortcuts but nothing happens when I press Caps Lock or Shift+Caps Lock. At the same time I can assign other keys, for instance Shift + Alt + {

I also checked the dconf-editor options in /org/gnome/desktop/input-sources/ and it seems that all options are included.