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.
