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).