If you already filed a return with only the 2 W-2s on it, you cannot simply add another one now. You would need to wait until you have received the refund on the earlier return before you make any changes to your return or file an amended return. Amend returns do not "catch up" to the original and change it. Amended returns have to be filed on paper by mail and it takes the IRS 12-16 weeks to process them. The amend form will be ready mid-February.
You also can't just look at how your refund changes. Many other things change in your return from adding a W-2. Your income goes up and your tax withheld goes up. If the right amount is withheld, then the additional tax you would owe from the income is matched by your additional withholding and you will see no change. You should print your original return before adding the W-2, then print the amended return after you prepare it to see what has changed.
Following are instructions.
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed#
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.