1. Use the IRS withholding calculator at
https://www.irs.gov/individuals/irs-withholding-calculator which does have ability to address multiple jobs and income sources.
2. Use the IRS Publication 15 and 15-A
https://www.irs.gov/pub/irs-pdf/p15.pdf and
https://www.irs.gov/pub/irs-prior/p15a--2019.pdf to calculate what each W-4 form will result in withholding, and then adjust the form values as needed so the right total amount is taken out.
