What's the reading of 二人 in this sentence?

Question

The sentence is

二人間がしっくり行かない

i'm confused when to use ににん and when to use ふたり

Answer 1

First, your example is likely a mistake for

二人の間がしっくり行かない

The original is substandard, if not wrong. And yes, 「二人」is pronounced「ふたり」in this case.

So, when is「二人」pronounced「ににん」? I don't think it's easy to come up with a simple set of rules, but at least if the sequence . . . 五人、四人、三人、二人 works down to 2, then it's pronounced「ににん」. For example, 「二人前」(two persons' portion). Why? Because in today's Japanese, the Chinese-based pronunciation (に、さん、し、ご、. . . ) is used for the number when the word is regarded as number + units, so "three persons", "five kilometers", etc. are pronounced さんにん、ごキロメートル, etc. On the other hand, 「ふたり」is no longer regarded as number plus units. (It historically is, so there used to be みたり、よたり、いつたり、むたり、after ひとり and ふたり.)

「ひとり」is a bit different. There still is a strong tendency to prefer ひとり to いちにん. So, 「一人前」can be pronounced ひとりまえ.

Finally, what's wrong with「二人間」? Isn't it like「日米間｣? Well, I don't have a logical answer to it, but we simply don't say ｢二人間」. Perhaps that's because that would require the Chinese-based pronunciation ににん in order for it to precede 間, but when talking about two persons we almost always refer to ふたり, not ににん.

Answer 2

It's ふたり. You always use ふたり when indicating a group of two people or their relation. Note that you can use both ふたり and ににん when just counting people.
One can tell 二人 in this sentence is not for counting number because of 間. It reads かん and means relation.

二人間: relation between those two
日米間: Japan-America relation
先生と学生間: between a teach and a student
夫婦間: between a married couple