You can have multiple sources of income and they all should be included on your tax return(2). If you are manually preparing a tax return you will need to add each box of the W-2's together: income, each category of taxes, etc.
If you use a tax program you will have the opportunity to enter a second employer and all of the information separately for each - no math required :). If information is missing from prior year returns you should consider filing an amended return for those year(2).
Following is a link with information to help you: https://turbotax.intuit.com/tax-tips/jobs-and-career/video-tax-tips-when-filing-for-multiple-jobs/L7...
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