In 1983, a similar OSHA Emergency Temporary Standard regarding asbestos was struck down.
The requirement for an ETS is that "employees are exposed to grave danger from exposure to substances or agents determined to be toxic or physically harmful or from new hazards" and that "such emergency standard is necessary to protect employees from such danger."
If Joe is vaccinated, he is not at "grave danger" from COVID, even if his colleague, Sally, is not vaccinated. Therefore, forcing Sally to get vaccinated fails this test.