H1B Transfer: How long employee can continue working with old employer?

I heard that, employee must join the petitioning employer within 60 days of the start of validity of the H-1B petition that was approved. This means that employer should resign the old company within 60 days of transfer petition approval. Is it true? Can anyone please confirm?

This is not mandatory. The petition is really not “transferred”. Once the new petition is approved, you end up w/ 2 petitions approved for 2 different employers. You can decide to stick w/ old employer or start working for new employer. It’s your choice.