It is possible to say: "Das ist zwischen mir und dir. Das ist zwischen mir, dir und ihm.", but I have never heard anybody say: “Das ist unter mir und dir”.
Therefore I would say that “zwischen uns” is used for two, eventually three definited persons, while “unter uns” is used for a group.
In English there is a transition from between to among depending on the context. Does zwischen cover the meanings of both among and between? If you look it up, you can see there are cases where one can use only between or only among or either one. Anyways, there is a continuum of usage for between and among in English. It would help a lot if someone teaches me if zwischen can be used both as among and between.
I would like to know this too. In English, "between" is when something separates or connects just two things, and "among" is either 1) when something separates or connects MORE than two things or 2) when something is surrounding or permeating two or more things. So, the we keep the secrets "between" us two, but we keep the other secrets "among" us three, and in both cases if there are people with us that do not know the secrets, outsiders are "among" us.
Does German have similar distinctions?
Your first usage is not quite right. Among is used when there is an undifferentiated group of more than two, between when the parties are differentiated. Thus, let's keep this between us (you and me), let's keep this amount ourselves (referring to a group larger than two), but let's keep this between you, me, Bill and Tom (differentiated group).
But I have the same question about whether German has the same distinctions.
so i am a genius. I guessed that this is where betwixt comes from. And I am correct. gotta love being a germanic speaker!
Etymology From Middle English bitwixe, from Old English betwēox, from Proto-Germanic twiskaz (“twofold, double”), from Proto-Indo-European dwís (“twice, doubly; in two”). Compare Saterland Frisian twiske (“between”), Dutch tussen, German zwischen.