I think that if there are two drinking fountains next to each other, there should be just one line behind them, instead of two lines. If one line is moving faster than the other, then the people in the other line get cranky. If there were to be just one line behind both of the fountains, then the next person in line would go to the next available fountain. That way, the people who had been waiting the longest would get the drinks first.
And the water is free, except the fountains are in schools paid by our tax money.