If you review the full W-4 from with all 4 pages there is an adjustment for couples with two jobs. You should perform the calculations one time, and then submit a W-4 for each job based on the recommendations. Or, you can use the online calculator as indicated.
If you want to see exactly how the withholding is calculated by the employer, you can review publication 15-T. Basically, the calculation looks at your wages as if is was for a full year. Suppose you get paid $1000 per week. x54 equals $52,000 per year. Minus the $27,000 standard deduction is $27,000, and the tax on that is about $3000 (these are rough estimates). $3000 ÷ 52 weeks is $57 so that would be your withholding. If you earned $2000 the next week, the calculation would be repeated except that your tax would be $177, because $2000 per week ($104,000 per year) bumps you into a higher bracket.
