Solved by using aliases and unique key (email)

seed.graphql

mutation{
  speaker: createSpeaker(data: {
    name: "Name"
    email: "example@example.com"
    workplace_url: "some_workplace_url"
  }){
    id
  }
  
  talkOne: createTalk(data: {
    speaker: {
      connect: {
        email: "example@example.com"
      }
    }
    
    title: "Test talk one"
    session_duration: 45
  }){
    id
    title
    session_duration
    speaker {
      name
    }
  }
  
  talkTwo: createTalk(data: {
    speaker: {
      connect: {
        email: "example@example.com"
      }
    }
    
    title: "Test talk two"
    session_duration: 30
  }){
    id
    title
    session_duration
    speaker {
      name
    }
  }
}