Although you may be claiming your 20 year old daughter as a dependent on your joint return, you will only get a $500 credit for other dependents for her; she is too old for the child tax credit. So listing her on only one W-4 or on both will have only a minor effect either way.
Use the IRS withholding estimator to help you.
https://turbotax.intuit.com/tax-tools/calculators/w4/
https://www.irs.gov/individuals/tax-withholding-estimator
https://www.irs.gov/pub/irs-pdf/fw4.pdf
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
