Most native English speakers will, I believe, regard
'We have known each other since ten years ago' as wrong or at least unidiomatic. It is not what people say to express how long they have known each other. We say either
'We have known each other for ten years' or
'We have known each other since 1993'.
There is in my view a good reason to regard the suspect sentence as incorrect. The tense implication of 'since' is opposite to the tense implication of 'ago'. 'Since' creates a present context, whereas 'ago' creates a past context.
As
soundshift correctly points out:
"We knew each other ten years ago" implies that we don't know each other now.
That is what a past context does: it disconnects the past event (or state) from the present.
'Since' does the opposite: it connects the past event to the present. 'He has been the mayor since 2007' tells us that he is still the mayor now. 'He was the mayor ten years ago' tells us that he is not the mayor now.
It is therefore contradictory to try to bring the word 'ago' into a sentence which requires a present context.
To express the proposition that we have known each other for a number of years and still do so now requires a verb with a present context and a sentence with a present context.
'We have known each other for ten years' does exactly that: and so does
'We have known each other since 1993'. However, it is incorrect to bring in a word which creates a past context, because that would imply a break between past and present.
That break between past and present is seen in the sentence
'We knew each other ten years ago'. This implies we no longer know each other: we have moved apart emotionally, or have followed completely different paths in work and life, so that we are practically strangers today.