Withholding for married couples can be tricky and depends on many factors including the desired refund amount if any. Use the W-4 worksheet (look to page 2) or an online tool to compute the correct figures for you both.
http://www.irs.gov/pub/irs-pdf/fw4.pdf
http://www.irs.gov/Individuals/IRS-Withholding-Calculator
https://turbotax.intuit.com/tax-tools/calculators/w4/
