(c) Exceptions for certain business or rental use; limitation on deductions for such use
(1) Certain business use
Subsection (a) shall not apply to any item to the extent such item is allocable to a portion of the dwelling unit which is exclusively used on a regular basis—
(A) as the principal place of business for any trade or business of the taxpayer,
How do I apply it to these facts? Unmarried couple AB rent a house and use it as their residence. A uses one room exclusively in his Sch C business B uses the same room exclusively in her Sch C business. No one uses that room for personal purposes.