SALT limit only applies to Sch A itemized deductions, it should not apply to form 8829. You do not split the property tax if there are 2 schedule C, you apply the full property taxes on form 8829 and appropriate property tax deduction is used based on percentage use of the home office. These allocated property tax deductions are then deducted from Schedule A.
Turbo tax does the calculations correctly where it reduces the amount of property tax claimed in Schedule C with form 8829 - use of home office.
However, there is a bug in turbo tax where it adds the total property taxes across all Schedule Cs. That is wrong. For example if you have total property tax of $20,000 and you have 3 sch Cs where each business uses 5% of your home space. You will use 20,000 as property tax for all 3 Sch Cs. Turbotax will calculate $1000 as tax deduction for each Sch C. The property tax on Sch A will now show $17000 ($20000 less $3000).
This is totally fine. $3000 claim on property tax for business is allowed and your property tax deduction of $20,000 is still below SALT limit.
However, Turbo tax shows SALT error on form 8829 saying that you cannot claim more than $40000 due to SALT cap. It thinks that you are claiming $60000 in property taxes. It also calculates wrong Schedule A in auto mode where it shows $60000 in property taxes in Schedule A worksheet rather than $20000. You have to change than manually to $20000 for the sake of this example.
I hope Turbo Tax fixes this issue for form 8829 as it is currently showing error.
