I would like to execute 2 stages on 2 different nodes. First stage should be executed always, and the second under a certain condition.

Everything works, except that when the second stage is not executed (the 'when' expression is not satisfied), a checkout of the repository is still done. 

This is the Jenkinsfile:

pipeline {
  agent { label 'node_1' }

  stages {
    stage("Stage 1") {
      steps {
        script {
          echo "Stage 1"
        }
      }
    }

    stage ('Stage 2') {
      when {
        expression { env.BRANCH_NAME == "master" }
      }
      agent { label 'node_2' }
      steps {
        script {
         echo "Stage 2"
       }
     }
   }
  }
}

The big problem with this is that in case the 'node_2' is executing other jobs, the current job cannot continue until this node has a free executor.