Login script that checks two users and their corresponding passwords
This is tagged programming-challenge, but just to be sure it's said: This obviously is not secure in any sense; the passwords are in the source.
Anyway, I'm assuming security isn't the point.

First of all: Spellcheck. "useranme" and "Your not chip/monty".

Secondly, my question is "why 2 users?" What if it's 3? Or 6,123,345? Right now you'd have to duplicate everything in your code to accommodate more users. Obviously that'll be extremely cumbersome.

A much better approach is to write code to check any given user/password against a list of users (rather than named variables).

So, first we'll need a list of users (or, in this case, a JS object):

var users = {
  'Monty': 'Cheese',
  'Chip':  'Gadget'
};


Then, some login logic (this can be a function, or several functions, or many other things - this is just a simple implementation)

var username = prompt('Enter Username:', 'username');
if( !username ) { // if username is blank, null or otherwise false'y
  return; // stop here if no name is given
}

// alert and stop if the username doesn't exists
if( !users.hasOwnProperty(username) ) {
  alert('Unknown username');
  return;
}

// now we can get and check the password
var password = prompt('Enter Password:','password');
if( users[username] === password ) {
  document.write('&lt;h1&gt;Welcome to the site ' + username + '&lt;/h1&gt;');
} else {
  alert("You're not " + username + "!");
}


And done.