[RealJames]

Between the two of us:
- there are only two of us
- there are many of us but only the speaker and the person he is speaking to
ex: "Hey Mark, between the two of us we can buy a PS3."

Between two of us:
- any two of the people in a group of more people
ex: (imagine there are 6 people there) "We should be able to cover the bill between two of us." (which two isn't specific)