What I’m talking about is what is listed on page 2 under specific instructions, step 2, option C.
https://www.irs.gov/pub/irs-pdf/fw4.pdf
Specific Instructions
Step 1(c). Check your anticipated filing status. This will
determine the standard deduction and tax rates used to
compute your withholding.
Step 2. Use this step if you (1) have more than one job at the
same time, or (2) are married filing jointly and you and your
spouse both work.
Option (a) most accurately calculates the additional tax
you need to have withheld, while option (b) does so with a
little less accuracy.
Instead, if you (and your spouse) have a total of only two
jobs, you may check the box in option (c). The box must also
be checked on the Form W-4 for the other job. If the box is
checked, the standard deduction and tax brackets will be
cut in half for each job to calculate withholding. This option
is accurate for jobs with similar pay; otherwise, more tax
than necessary may be withheld, and this extra amount will
be larger the greater the difference in pay is between the two
jobs.
All I know, is the accountant said this would have saved us $2-3K in taxes.