Execute when both(!) events fire .on('end')

Question

I have a node app that reads two files as streams. I use event.on('end') to then work with the results. The problem is I don't really know how I can wait for BOTH events to trigger 'end'.

What I have now is:

    reader1.on('end', function(){
        reader2.on('end',function(){ 
          doSomething();
        });
    }); 

With small files this works, but if one of the files is very large the app aborts.