use a typed expression with a scalable item.

				
					    "id":"_ID_",
    "jsonrpc":"2.0",
    "method":"hub.scenes.expressions.set",
    "params":{
        "name": "kitchenHallTempAverage",
        "params": {
            "items": [
                {
                    "name": "kitchenTemperatureSetpoint",
                    "_id": "5fecdf73222aaa175f6d6be4"
                },
                {
                    "name": "hallTemperatureSetpoint",
                    "_id": "60c7696b933ffb710a8be319"
                }
            ]
        },
        "code": "return { value = (params.kitchenTemperatureSetpoint.value + params.hallTemperatureSetpoint) / 2, scale = params.hallTemperatureSetpoint.scale }",
        "valueType": "temperature",
        "metadata": {},
        "variable": false
    }
}