We have one hundred dollars"between"us.我们两人共有一百块钱

回答：
您举的例子中的between是介词
介词 prep.
1.(指时间、空间、顺序等)在...之间
They planted a lot of trees between the two buildings.
他们在两座建筑物之间栽了许多树.
These books were written between 1736 and 1770.
这些书写于一七三六至一七七零年之间.2.(指数量、距离、程度等)介乎...之间
He is a man between sixty and seventy.
他的年龄约在六十到七十之间.3.连接着...;来往于...之间
There is a path between the house and the road.
有条小路将房子与出路连接起来.4.为...所分享(或分担)
Between them they landed the big fish.
他们齐心协力将大鱼捕上岸.
The three children saved about two hundred dollars between them.
三个孩子一起积蓄了大约二百元钱.5.由于...的共同影响
Between work and studies Bill has no time left.
工作和学习使比尔无暇它顾.
副词 ad.
1.在中间;介乎两者之间
We had a faculty meeting at 2 p.m.and a lecture later,with a 15-minute coffee break between.
午后二时我们开教师会议,而后有一个讲座,在此之间我们有十五分钟喝咖啡休息.

意思：在我们俩者之间
因为是俩者间所以是between A and B.
拓展：among是三者之间的