While it could be done, why does he want to do that?

From a business perspective, it makes sense to know how each business is doing by keeping track of the income and expenses separately from other businesses.  That way they can analyze how each business is doing.

If everything is being tracked separately, what is the purpose of then lumping them together on the tax return?   It could also hypothetically increase audit risk if the expenses are significantly off from the 'usual' expenses for the type of business code.