H-1B Transfer-do I need to resign now?

I got a new job recently. The new firm is applying for the H1B transfer for me. Since it will take two-three weeks for the premium transfer, do I need to resign in order for the new firm to transfer my visa? Or can I still with the old firm until the application for transfer is approved?

I think this question is equivalent to: can I have two H1B at the same time?

Thanks a million!