Send messages from Jenkins to multiple Slack channels
You can add channel: '#CHANNEL_NAME:
More info: https://jenkins.io/doc/pipeline/steps/slack/
post { success { slackSend (channel: '#ch1', color: '#00FF00', message: "SUCCESSFUL: Job '${env.JOB_NAME} [${env.BUILD_NUMBER}]' (${env.BUILD_URL})") } failure { slackSend (channel: '#ch1', color: '#FF0000', message: "FAILED: Job '${env.JOB_NAME} [${env.BUILD_NUMBER}]' (${env.BUILD_URL})") }}