I am a CPA and my wife is an IRS auditor, so I think I have a good answer for you
The W4 is used to determine the amount your company withholds based on your salary - pretax deductions (health ins, 401k contributions, etc). The idea is to fill out your W4 as you will be claiming on your taxes: if you will file single fill out your W4 as single, if you will file as married fill out the W4 as married, if you will claim 2 spouses and no kids each should claim 1 exemption on the W4.
The problem that many people run into is that the W4's formula is based on the olden days: days when the Man made significantly more than the Woman. So the W4 formula for married people who make the same amount of money results in massive underwithholding.
So what should someone do in this situation? If both spouses work and each spouse makes "real money" (more than $25,000) the withholding schedule won't work right on its own. To be safe you can do two things:
1) Have the lower earning spouse claim Single and 0 allowances and the higher earning spouse claim Married and 0 allowances. This should get you fairly close
2) Use Google to get the tax rate schedule and just do the math. If you can project your income and deductions you can get your taxable income. Then you can use your projected taxable income to compute your tax liability from the tax rate schedule. Then you look at your pay stubs and see how much you have withheld so far this year, how much is being withheld each check, how many checks you have left and you just do the math to see how much you will have withheld at year end (amount withheld already + (amount withheld each check X # of checks remaining) = total amount withheld this year). If you are underwithholding you can adjust your W4 and you can even have your company withhold more money. If your company won't withhold more money you can make estimated tax payments to the IRS on a quarterly basis.
The tax rate schedule is the thing that looks like this:
Married Filing Jointly or Qualifying Widow(er) Filing Status
10% on taxable income from $0 to $17,850, plus
15% on taxable income over $17,850 to $72,500, plus
25% on taxable income over $72,500 to $146,400, plus
28% on taxable income over $146,400 to $223,050, plus
33% on taxable income over $223,050 to $398,350, plus
35% on taxable income over $398,350 to $450,000, plus
39.6% on taxable income over $450,000.