I'd argue yes, they should definitely still be employees.

When a person flips burgers for both McDonalds and Burger King, they're still clearly an employee with two different jobs at two companies, and not some sort of "independent food consultant". If you work as a cashier at both Shell and Speedway, you are today an employee with two different jobs at two different companies, and not some kind of "independent petroleum cashier contractor". Having two jobs at different companies happens all the time in the US today for low/middle-class folks, and has for many many decades now -- these folks are generally all still considered W2 employees.

It is not uncommon to have multiple employers and be an employee at all of them. Someone might work one shift at CVS, then another at Rite Aid. Someone might work at Wendy's one day and work at McDonald's the next.

If Apple or Google are ok with that then there is no legal reason they couldn’t. Of course “works somewhere else” isn’t a protected class, so how those employers respond to that arrangement is entirely up to them.