Karabiner rule that switches to MigrantTastatur on Caps and to Russian on Shift-Caps

srcswitch.json

{
    "title": "Sitnik-style Source Switching",
    "rules": [
        {
            "description": "Caps Lock to en, Shift+Caps Lock to ru",
            "manipulators": [
                {
                    "from": {
                        "key_code": "caps_lock",
                        "modifiers": {
                            "mandatory": [
                                "shift"
                            ]
                        }
                    },
                    "to": [
                        {
                            "select_input_source": {
                                "input_source_id": "^com\\.apple\\.keylayout\\.Russian$"
                            }
                        }
                    ],
                    "type": "basic"
                },
                {
                    "from": {
                        "key_code": "caps_lock",
                        "modifiers": {
                            "optional": [
                                "any"
                            ]
                        }
                    },
                    "to": [
                        {
                            "select_input_source": {
                                "input_source_id": "ru.karamoff.keyboardlayout.migranttastatur"
                            }
                        }
                    ],
                    "type": "basic"
                }
            ]
        }
    ]
}

Inspired by @ai

Uses MigrantTastatur as primary input source.