Dear TJE,
Being an H1B employee you should only work for 1 employer who runs your payroll. Incase if you want to work for company B. then company B should file a separate H1B application behalf of you but here comes the real tricky part the company B’s H1B must be approved for 20hours a week this is the only way you can work for 2 companies. but personally i suggest you not to risk your carrier, stick to one employer. Takecare.