I would presume it would work the same as if you had two other part-time jobs, such as working at both Walmart and Best Buy.
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.