Each employer withholds as if that was the only income, but when added together you can move into a higher tax bracket so more withholding (or paying estimated tax) is necessary. When both spouses work they might need to claim single and not exemptions to avoid paying additions tax or even having an addition amount withheld. This is very common with two incomes.
See this FAQ:
Why did my refund drop when I entered another W-2?
https://ttlc.intuit.com/replies/3798403
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
