How much to withhold for married, 2 kids earning 300k?

We have two kids, and we both are fortunate to earn around 150k a year. When filling out a w2, do we fill one together to work out the $ value to tell our separate employers to deduct. I take it that we should have 4 deductions (one for each family member) and file married when calculating our withholding on a calculator. My question is how many exemptions should we have, and status should we withold at, and do we need to withold additional amounts?