Huh? I just said that would largely not be considered tolerable. Imagine working at a food court as a flex hours worker and jumping between the KFC counter and the McDonalds counter during the same shift. That might maybe sort of work if only one person does it, but that scheme is going to fall apart on the first lunch hour if all your employees are flex.

What employer would allow that?

Of course an employee can work at KFC and McD simultaneously if they can make arrangements to not have shift conflicts, and likewise one could hypothetically be a Uber employee in the morning and a Lyft employee at night, but why go through the trouble of working effectively full time but only getting redundant part timer benefit packages?