If you structure your business in such a way that it is simply a direct sales sole proprietorship, then it would make sense to combine your income and expenses on a single Schedule C. This especially makes sense if you have similar or shared expenses for both direct sales companies.
If representing the two direct sales companies resulted in substantially different types of operations and different types of expenses that would not intermingle, then they should be reported separately using two Schedule Cs.
Will having one 1099 from each of the two separate companies, but having expenses combined, flag an error or cause for audit, though? I'm in the same situation (ie working for two different direct sales companies) and don't want to cause an audit due to things looking peculiar...
Consider the situation if you separated the two into two different Schedule Cs -- would you use the same office space for both? Would you use the same supplies? Would you use the same vehicle? Or would you use a home office for one Schedule C and only use your vehicle for one Schedule C -- therefore not intermingling your expenses for both Schedule Cs.
To me, it is less likely to cause a red flag if the two are combined when the same resources will be used for both "businesses". Otherwise, you will need to maintain meticulous records to separate your expenses for each Schedule C.