他们俩彼此心照不宣。

They have a secret understanding between them.

Source: Tatoeba

Sentence Composition

1 他们俩 tāmenliǎ the two of them; those two people
2 彼此 bǐcǐ each other; one another
3 心照不宣 xīnzhàobùxuān to have a tacit understanding