I think here, "on the clock" also implies "available to drive" or "driving". I can't be driving two different people for two different companies at the same time, so I'm not really available to drive for both companies.

Because if they are driving someone for Uber, they can't be on the clock for Lyft.

But it is (physically) possible to be idle for both employers at the same time.

What happens if ride requests for both come in at the same time? Do you take the uber one or the lyft one? If you work for uber, you're obligated to take the uber ride. Ditto for lyft.

But it is possible to work out, technically.

Of course these companies prefer not to, because their business model is now based on exploiting workers.

This would require the employers to collude as to where to have you idling. (Employees don't get to pick exactly where they work.)

How so? That may be part of a given employment contract, but nothing in the legal definition of employment says it.

It's not part of the legal definition, it's a practical matter.

Assume the employers did not collude for an employee on shift at both employers during a given time period (say 9am-1pm):

- Uber says to idle near LAX, because that fits what Uber needs at that time.

- Lyft says to idle near BUR, because that fits what Lyft needs at that time.

Those locations are approximately 37 minutes away as I write this, despite both being quite logically in the same metro area. It's eminently reasonable to expect an employee to serve an area that encompasses (even much more than) the area between those two points, so an employee would not reasonably get a routine say in where inside the region she is asked to idle.

QED the only way to routinely guarantee that Uber and Lyft ask an employee to idle in the same location is for Uber and Lyft to collude on staffing decisions. This opens up other legal concerns (and may be illegal).