Currently it is possible to run only 1 stage into a thread inside a 'parallel' section. It is reasonable (and really required for my project now) to have a way to run several stages per thread into а parallel section like this:

-----------A-------B------------
     \                   /
      -----C-------D-----

For now the pipeline showed above is not possible besides running all the commands withing 1 stage in several steps.

It could be implemented like this:

stage("Parallel stage"){
    parallel{
        stages{
            stage("A"){...}
            stage("B"){...}
        }
        stages{
            stage('C'){...}
            stage('D'){...}
        }
    }
}