@John Smith says hello

John smith says hello    OR  simply 'John Smith' - Either is okay for this purpose

I was talking to @John Smith and he told me all about @Sarah Smith

Sarah Smith

txt.substring(txt.lastIndexOf('@')+1).split(' ').slice(0, 2).join(' ')

var a = "@tony @stark"
> undefined
var b = a.lastIndexOf('@')
> undefined
a.substr(b, a.length)
> "@stark"

a.substr(b+1, a.length)
> "stark"

function getRest(a) {
  var b = a.lastIndexOf('@');
  return a.substr(b+1, a.length);
}
getRest('@John Smith says hello')
> "John Smith says hello"
getRest('I was talking to @John Smith and he told me all about @Sarah Smith')
> "Sarah Smith"

var regex = /@(w+ w+)[^@]*$/,
    myString = "@spork and @abe lincoln spam spam spam",
    match = myString.match(regex)[1];
// match == "abe lincoln"