How does this work if they drive for both Uber and Lyft?
For all of the phoniness behind phrases like "ride-sharing" and "gig economy", workers have never had a more horizontal relationship with their bosses -- except, perhaps, in higher pay-grade service sectors where "bosses" are referred to as "clients".
It's extremely common to not have fixed shifts. You ask for hours, and you get what you get. It's really not that different from accepting individual driving jobs.