You wouldn't expect two separate W-2's from the same employer simply because your work hours changed. Have you finished your tax return? Don't assume you owe taxes until you've entered all your information and your return is completed.
If you finish your return and you find that you do owe taxes, it means that you did not have enough taxes withheld from your pay.
You can adjust your withholding at any time by filing a new W-4 with your employer. https://www.irs.gov/pub/irs-pdf/fw4.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
