You will need to amend the 2018 tax return to add the additional W-2. An amended federal tax return, Form 1040X, can only be printed and mailed, it cannot be e-filed. The IRS will take up to 4 months or longer to process the amended tax return. If taxes are owed after completing the amended return, you send a check with the amended return when it is mailed. If the amended return has a tax refund, then the IRS will send you a refund check in the mail for the refund, the refund cannot be direct deposited.
The IRS Form 1040X is estimated to be available on 02/22/2019. See this TurboTax website for IRS forms availability - https://care-cdn.prodsupportsite.a.intuit.com/forms-availability/turbotax_fed_online_individual.html
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
