Run at https://observable-playground.github.io/gist/f0c8048d10c0fe42f1e52220ba73eae5

index.js

const { chart } = require('rp-api');
const { forkJoin, of } = require('rxjs');
const { delay, tap, switchMap, mapTo } = require('rxjs/operators');

mockRequest(10, 'cid').pipe(
  switchMap(id =>
    forkJoin(
      mockSendQueue([1, 2], id),
      mockRequest(10, 'sub').pipe(
        switchMap(subid =>
          mockSendQueue([3,4], subid)
        )
      )
    )
  )
  , mapTo('done')
)
.subscribe(chart.createRxObserver());
  
// will emit response after time
function mockRequest (time, response) {
  return of(response).pipe(
    delay(time)
    , tap(()=>console.log(Date.now(), response))
    , tap(chart.createRxObserver())
  );
}

// will emit every value in the list
// with delay equal to value
function mockSendQueue (list, suffix) {
  return forkJoin(
    ...list
      .map(value =>
        mockRequest(value, suffix + ':' + value))
  );
}