The mortgage interest deduction is the amount of the total prorated to the square footage percentage used for the home office. It should not be the total mortgage interest paid. The amount you claim on Schedule A as an itemized deduction (if you itemize) is reduce by the amount deducted on the Schedule C.
When splitting the Home Office Deduction between the two home office schedules, in your example, you must use a percentage of the total expenses that apply to the home office space, THEN divide based on time used for each business.
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