How does the H1-B "transfer" / Multiple H1-B actually work?

[ajaysharma]

So I know that there's no such thing as a 'transfer' and it's just a new petition - but that's not really my question.

Can one hold multiple valid H1's, running concurrently, but employed in one and not the other, until the time of transfer?

I'm currently with employer A. But in the next few months I may be interested in moving on to something new.

Let's say I get an offer from B, on Aug 1 2017. I accept the offer, they process premium, and the H1 gets approved as of Aug 30, active from that day. However, I may still be with A, and give them a notice of a few weeks, and my final end date is Sep 30. I join B on Oct 1st.

Is that valid? In the past, whenever I've changed employers, it's always been that the approval has been after my previous employment ended.