I am starting a side business from a hobby. My main business is an LLC filing as an S-Corp and run from my home. My side business is a Sole-Proprietorship. Can I use some of the same equipment for both businesses and how do I account for this? Office space, computers, printers, desks, car mileage, etc? I know of people running multiple businesses and can't imagine they own separate computers, desks, etc, for all of them.
It is expected that you should allocate expenses to the particular activity in a reasonable and consistent manner. The car mileage should be pretty straight forward, just track the miles for each business in separate logs and reimburse accordingly. Some deductions may not be as easy to determine or split. If the allocation to one or the other is very minimal it may not be worth apportioning. But if you can determine a fair percentage and it is material, you should allocate the expenses. If one entity is paying for the expense just have the other entity reimburse for its share.