combined_obs.js

const session$ = function (key) {
  const request = request('GET', 'YOUR_URL')
  return Rx.Observable.create(observer) {
    req.on('response', (res) => {
      // check status code
      // obs.onError() if not 200
      // else...
      res.on('end', () => {
        let result = ''
        res.on('data', (chunk) => {
            result += chunk
        })
        const json = JSON.parse(result)
        observer.onNext(json)
        observer.onCompleted()
      })    
    })
  }
}

const data$ = function (token) {
  const request = request('GET', 'YOUR_URL')
  return Rx.Observable.create(observer) {
    req.on('response', (res) => {
      // check status code
      // obs.onError() if not 200
      // else...
      res.on('end', () => {
        let result = ''
        res.on('data', (chunk) => {
            result += chunk
        })
        const json = JSON.parse(result)
        observer.onNext(json)
        observer.onCompleted()
      })    
    })
  }
}


const combinedResult = session$(key)
  .flatMap((sessionResult) => {

    console.log(`Session Token: ${sessionResult}`);
    // use this for your next call....

    // you probably want to transform your data ?
    const dataSignal$ = data$(sessionResult)
      .map((data) => {
        return {
          title: data.title
        }
      })

    return dataSignal$
    
  })
  .subscribe(
    (dataResult) => console.log(`data: ${dataResult}`);
  )