The US tax system doesn't work that way. Individuals (or joint filers) are taxed on the total taxable income and the tax rates are marginal. The way you are currently working your returns is as close as you will get.
Here is an example. For joint filers, income from zero to $19,750 is taxed at 10%. Income between $19750 and $80,250 is taxed at 12%.
So if a taxpayer first enters 17,000 of W2 income that will be taxed 10%. Then the taxpayer enters $17K of a business income. The first $2750 is taxed at 10% and the rest at 12%. Then the tax payer enters $17K for the second business all of that is taxed at 12%.
But if you change the order of entry the tax on each entity changes. Also, if taken separately each entity would be taxed at 10%.
Also, if you have capital gains from dividends or selling investments, that income is taxed at different rates as well.
Perhaps one way to work it is to take each business income separately and determine it's tax based on the tax brackets. The W2 income will then be the one that is absorbing all the marginal taxes.
Here is a link with the tax brackets if you need more information.
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