It is important to understand that the W-4 generates an estimate of your income taxes that your employer uses to determine your withholding. For a single person, that includes the assumption that they qualify for a $13,850 standard deduction (in other words, the first $13,850 per year or $266 per week is tax-free). If you work 2 jobs and don't make adjustments, each W-4 will assume you have a $13,850 standard deduction, so you will be under-withheld by as much as $5000 over the course of a year. That will result in a difficult tax bill to pay plus late fees and interest.
You should carefully read and follow the instructions on all 4 pages of the W-4 and prepare both forms at the same time following one set of instructions. If you have too much withheld, you will get the difference back as your tax refund.
