What happens and what to do if they purchased their home during 2019 and their mortgage debt is $2M?

I want to say I could just put the entire mortgage interest on schedule E and let it pro-rate to schedule A based on rental %.  I wonder if this is allowed as it's partly for business usage, meaning do I have to apply the $750K limit.  If so how should this be done?

A corporation is renting out part of their home as office for one of the shareholders.