If a nanny is married and both spouses work and both spouses had their withholding tax adjusted, they are being credited for 150% of the available credit - and may be surprised to owe several hundred dollars at the end of the year. If you work two or more jobs and have withholding from each, you may be being under-withheld because each job treats your withholding tax calculation as if you are working only for them. Nannies in a nanny share have to be particularly vigilant that both jobs are taking into account your TOTAL income, not just the portion they pay individually, when calculating your withholding.
Remember, withholding tax calculations are simply estimates. You true tax calculations depend on many factors other than your income and marital status.
The IRS withholding calculator at IRS.gov and Publication 919, How Do I Adjust My Withholding?, can help a taxpayer compute the proper tax withholding.