present perfect tense subject + have( i, we, they ,you and with pural )or has( he , she, it, with name and with singular)+ 3rd form of verb+ object. in other way also we use present perfect tense like that:- subject +has/have+ 3rd form of verb+ since/for+ object.

Although I am no expert on the subject, I am a native speaker of English. I feel that the difference between the 2 is that in the 1st example, the past tense is used first, i.e. "since I met her", because meeting somebody for the first time is a one-off, single occurrence; like a completed action. Whereas in the 2nd example we use the present perfect, "since we've known each other" because it is a continuing action or state of affairs because you continue to know somebody, but you can't continually meet somebody for the first time. To summarise, example 1 is about a completed, single action and therefore uses a normal past tense in the first clause, whilst example 2 is about a continuing action and thus requires the use of a present perfect in both clauses. I hope I have sufficiently cleared up any doubts or confusion that you had, and if you have any further questions please do not hesitate to ask me! James