How to get username password stored in Jenkins credentials separately in jenkinsfile

Here is a tiny bit simpler version of StephenKing's answer

withCredentials([usernamePassword(credentialsId: 'mycreds', usernameVariable: 'USERNAME', passwordVariable: 'PASSWORD')]) {
  sh 'cf login some.awesome.url -u $USERNAME -p $PASSWORD'

}

You can use the UsernamePasswordMultiBinding to get credential data in separate values:

withCredentials([[$class: 'UsernamePasswordMultiBinding', credentialsId:'mycreds',
  usernameVariable: 'USERNAME', passwordVariable: 'PASSWORD']
])

So the following should work in your case:

withCredentials([[$class: 'UsernamePasswordMultiBinding', credentialsId:'mycreds', usernameVariable: 'USERNAME', passwordVariable: 'PASSWORD']]) {
  sh 'cf login some.awesome.url -u $USERNAME -p $PASSWORD'
}