H-1B workers may be employed by more than 1 employer concurrently. A typical scenario is 1 full time employer and 1 part time employer.
If the H-1B worker will continue to work for their employer while commencing new employment for a second employer, the second employer must file an H-1B petition requesting extension of H-1B status of the worker.
Under the AC21 portability rules, the worker may start employment for the second employer as soon as the petition has been filed; there is no requirement to wait for approval of the 2nd concurrent H-1B petition.
The 2nd concurrent H-1B petition position must still be a specialty occupation requiring a relevant bachelor’s degree or foreign equivalent.