彼此【bǐcǐ】

彼此 means "each other". The subject is plural, and 彼此 sits before the verb like an adverb. It's neutral to written.

As an object

彼此 is also a pronoun, so it can follow the verb as its object.

彼此之间 is "between us, between them".

彼此 or 互相

互相 is the everyday word for "each other", but it's only an adverb: it goes before the verb and can't be an object.

• × 我们了解互相 → 我们了解彼此 / 我们互相了解

彼此彼此

Doubled and said on its own, 彼此彼此 answers a compliment or a complaint: "same to you", "we're in the same boat".