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 two or more jobs you might need to claim single or even have 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.**
