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?
generally i would recommend the one with the lower compensation to claim all dependent exemptions. say the higher income taxpayer claimed all. withholding would normally be a higher % of taxable wages of the higher income taxpayer. if that taxpayer were to claim then all the withholding could be reduced to the point that you are subject to underpayment penalties. however, this is just a generality.
what you could do to check is to use TT taxcaster to estimate your taxes. this wouldn't give a perfect estimate but it should be close enough. this is based on 2019 tax laws. there are proposals in Congress to further reduce taxes in 2020 for individuals but no one knows if this will ever pass
once you get your tax estimate you can use the TT withholding calculator and try various scenarios
understand these are only estimations. if your wages vary during the year you should revisit and recalculate
you can then submit new W-4's to change withholding
you can avoid penalties no matter what you tax is for 2020 if you meet any of these tests. I'm going to assume you are not going to pay estimates
1) withholding covers 90% of 2020 tax
2) withholding equals 100% of 2019 tax (110% if adjusted gross income over $150,000)
3) balance due after withholding is $1,000 or less