二位 = two people. 位 = polite counter for persons! That is why it is used in this setting.

The sentence "服务员说：二位要吃一点儿什么？" means "what would you two like to eat ?“二位”is the same meaning of " 你们”But use the "二位" is show respect .Usually use "二位" in restaurant ,hotel and so on ..For example ,when u go to the restaurant ,the waiter will say "先生几位 ？"means "How many people would in this dinner ,sir ?" The " 几位" is the same meaning of "多少人" , means "how many people ?"

“位”like the words "a man", and this word "位" can't use at the vais allons. “位”这个字可以解释为“个+人”，这个词不能用在第一人称。 ex. 服务员说：“请问您几位？”

I think Eliot's explanations are most correct. （两）位 just is a polite and commendatory term which is equal to "one" or is used for replacing two people. For example, 这位 means this(这） one（位）. 两位同志 two comrades, in this expression, 位 is a quantifier. It is like the use of piece when using a piece of paper in English.

位是数量词，例如：一个人，两个人，三个人，二匹马，三头牛，其中“人、匹、头”都是数量词。 二位=tow people,这里是较礼貌的用法。