"They are both equal to each other" or "They both equal to each other"?

What is the appropriate sentence? (Maybe something else?)

1. They are both equal to each other.
2. They both equal to each other.

NB: I'm not a native English speaker.

Of the two sentences you give They are both equal to each other is grammatical and They both equal to each other is not. "equal" in "equal to" is an adjective, so sentence 2 lacks a verb.

We can use equal as a verb, this would give the sentence They both equal each other.

They are both equal to each other and They both equal each other are grammatical and they express the same thing.

As others have noted there is redundancy in the above sentences, unless there is room for confusion a more concise sentence is preferable. - However without knowing more about context I would not state that sentence 1 is wrong.

Avoiding the redundancy is bad if there is room for confusion. For example if you are trying to make the following point; "Since a equals b we know that they both equal each other."