They certainly don't let you work for both at the exact same moment in time (which is quite common with drivers who have both Uber and Lyft open at same moment). Also Burger King generally sets your schedule and McDonalds usually sets your schedule. It's usually not feasible to work for both, because they both might end up scheduling you in the same time slot.
You are describing a literal impossibility as your example, being physically present at the Burger King and McDonald’s at the same time.
It’s the very same thing as say a driver can’t give a passenger a ride for Uber at the same time they are giving a ride for a Lyft passenger.
> It's usually not feasible to work for both, because they both might end up scheduling you in the same time slot.
You are essentially trying to argue no one Can work 2 jobs because of potential scheduling conflicts. Plenty of people work 2-3 jobs, often times for competitors in the same industry.
