// nest them

var a = 0
var b = 0

fs.readFile('filename.txt', function(err,data){
  a = data  
  fs.readFile('file2.txt', function(err,data){
    b = data
    console.log(a,b) // both values
  })
})

// use a library called async
// npm install --save async

var async = require('async')
var fns = [] // array of functions
var a = 0
var b = 0

// push a function on to the array
fns.push(function(done){
  fs.readFile('filename.txt', function(err,data){
    a = data  
    return done()  // a special async function that signals to move on
  })  
})

// push another function on to the array
fns.push(function(done){
  fs.readFile('file2.txt', function(err,data){
    b = data  
    return done()  // a special async function that signals to move on
  })  
})

// push the last function on to the array
fns.push(function(done){
  console.log(a,b) // both values are populated
  return done()
})

async.series(fns)  // this is where all the 3 functions you just pushed on to the array are run in order