node {
    stage("Build") {
        parallel([
            "Android":{ stage("Android"){ stage("a"){ echo "x" }; stage("a1"){ echo "x" }}},
            "iOS":{ stage("iOS"){stage("b"){ echo "x" }; stage("a2"){ echo "x" }}}
        ])
    }
}

pipeline {
    agent any
    stages {
        stage('Build') {
            failFast true
            parallel {
                stage('Android') {
                    stages {
                        stage('a') { steps { echo "x"} }
                        stage('a1') { steps { echo "x"} }
                    }
                }
                stage('iOS') {
                    stages {
                        stage('b') { steps { echo "x"} }
                        stage('b1') { steps { echo "x"} }
                    }
                }
            }
        }
    }
}