just the two of them
Emphasizes that only two people are present, often implying privacy or intimacy. Compare with 二人きり, which has the same meaning but without the geminated っ.
See also: 二人きり
二人っきりで話したいことがある。
There's something I want to talk about just between the two of us.
部屋に二人っきりにされた。
We were left alone together in the room.
Standard kanji spelling.
Alternate kanji spelling using 切.
Variant using numeral ２.
Variant using numeral ２ and kanji 切.
From 二人 (ふたり, 'two people') + っきり, an emphatic form of きり ('only, just'). The geminated っ adds emphasis.