Yes that can happen. The Pet Sitter who is providing care during an overnight stay may not be able to also handle the dog walking needs during the day.
Dog walkers are more apt to employ dog walkers! Because of the window of time when dogs need to be walked, a dog walker may quickly become over booked. Or, because of injury, illness, vacation or other event, the dog walker may need to hire a substitute. Either way, if you hire an employee there are additional requirements, such as providing workers compensation.