Thank you for your question, and thanks for using JustAnswer.com.
The correct thing to do is to issue two W-2 forms, one from each company. The IRS already has record of you paying your quarterly taxes on the first company and it will mess things up if you just issue one W-2 for both amounts.
So, 2 W-2 forms for each employee. This may involve some manual calculations, and your W-2s for your second company may need to be manually entered.
I hope this makes sense. Please let me know if you need further assistance. Thanks for using JustAnswer.com and have a happy, and safe, 2013.
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You should be aware that your question, as with most tax questions, can never really be answered completely...addressing all the permutations; that is because of the many assumptions that have to be made I have done my best to determine what I think you are asking and answering it in the most direct and understandable manner possible. If, however, after reviewing the questions, you have any uncertainties or further questions, please do not hesitate to ask.