If you have already filed your return you MUST WAIT until the IRS accepts your return. And, if you are due a refund it is best to wait until you have actually received your refund. Then, and only then you can prepare an amended return with the additional W-2s (forms) added. The amended return MUST be PRINTED and MAILED --it CANNOT be e-filed. An amended return generally can take 3-4 months for the IRS to process it --and the processing of amended returns does not start until later in the year.
If your return is rejected, you can add the additional W-2s (forms) at the same time as you make the corrections required by the IRS and re-submit your e-file.
Above all, DO NOT DO ANYTHING until your return is accepted or rejected.
When mailing a printed return it is always a good idea to go to the Post Office and mail it 'Certified' with 'Return Receipt'. That way you will have proof that they received it and when it was received.
