Qui est ce que vs qui est ce qui

Qui est-ce qui t'a parlé? -- (Who is that) who talked to you?
Here, qui is the subject of the relative clause and te (t') is the indirect object. In that case, always use qui.

Qui est-ce que tu as rencontré? -- (Who is that) whom you met?
Tu is the subject this time and que is the direct object. You always use que with the direct object.

Rather than try and then fail to explain adequately try this link

 Inverted questions with reflexive verbs in the present tense (Le Présent) in French%252Fsearch%253Fs%253DInterrogative%252Bpronouns%252B

and if that doesn't help then try to search again from that link  --  this issue has been discussed many times and is tricky to master.

Bonne Continuation