I still do not understand. In case I have a city with two neighbouring cities which belong to opponents. If the service point of my city is higher than the opponents' cities, should my city gets two citizens from them?
If your city is stronger than bot your opponents' then it will receive one citizen from each city when it's your turn to evaluate your cities.
Or, if the service point of my city is lower than the two cities, should I lose two citizens to both cities when the respective opponents evaluate their cities' service point? Thanks in advance.
That's correct. If your city is weaker, you'll lose 1 citizen to each of your opponents's cities, but on their turn to evaluate their cities.