No. You can never file 2 different years of W-2s on the same tax return. TurboTax Online is only for tax year 2019 and only W-2s from 2019 should be entered. You need the 2018 version of TurboTax to file your 2018 taxes. Or, if you filed a 2018 tax return last year, you would amend that return to add the 2018 W-2.
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