race.js

let {readFile} = require('fs')

let tally = {ab: 0, ba: 0}
let globalDone = 0
let COUNT = 1000

for (let x = 0; x < COUNT; x++) {

  let result = ''
  let done = 0

  let checkDone = () => {
    done++
    if (done === 2) {
      globalDone++
      tally[result]++
    }
    if (globalDone === COUNT) {
      console.log(JSON.stringify(tally, null, 2))
    }
  }

  readFile('./a.txt', {encoding: 'utf8'}, (error, data) => {
    result += data
    checkDone()
  })

  readFile('./b.txt', {encoding: 'utf8'}, (error, data) => {
    result += data
    checkDone()
  })
}

/*

Output from a few runs:

$ node ./index.js 
{
  "ab": 969,
  "ba": 31
}
{
  "ab": 988,
  "ba": 12
}
$ node ./index.js 
{
  "ab": 983,
  "ba": 17
}
$ node ./index.js 
{
  "ab": 970,
  "ba": 30
}
```