Not sure I understand your question correctly but I think you could emit value as an array of doc.City and doc.Value. In the reduce, you will return an array of combined doc.City and doc.Value. The group level will be one.
If you don’t plan to sort by doc.City, you could drop it from the map function key as well.
You cannot use _sum function to combine the array. You need to write a reduce function. In the reduce function, you could loop over the values which is an array of [city,value] to combine city (in the format that you want) and sum the value. The return value will be [combined cities, total value]