Your tax withholding is directed by the W-4 that you have completed and is reflected on your paycheck. You may check your withholding with your HR Department and you may change it at anytime throughout the year. The withholding each payday is calculated using your withholding status (married or single and the number of allowances) and the tables or formulas provided by the taxing authorities.
 

Multiple W-2s can result in increased tax liability since each individual employer may withhold taxes at a lower rate. For example, if you earn $30,000 from each of two employers, they might each withhold tax at the $30,000 tax rate when your true combined income places you in the $60,000 tax bracket. Additionally, if you earn income in more than one state, you may owe higher state taxes in one of the states.

You are more likely to need to increase your withholding if you have more than one job or if you are married filing jointly and your spouse also works. If this is the case you can increase your withholding for one or more of the jobs. The IRS withholding calculator can help you decide what to enter on your W-4, or to estimate the effect of changing your filing status or allowances.
 

You need to give your employer a new Form W-4 for each job for which you are changing your withholding.