Ask xiaobaitu and Martinlu experts, please. What's the start dat

Xiaobaitu and MartinLu experts,
I'm currently working full-time with company A and got another full-time job offer from company B. I'm going to apply concurrent H1B instead of transfer my H1b to company B. Can I start to work for company B the next day after I my concurrent H1B? Or I have to start to work after concurrent H1B got approved.Thanks a lot for your reply.