Urgent question about Cobra

I am told Cobra coverage ends if you take on new insurance, but an existing second insurance is ok. I am wondering if for the beginning of cobra, the determination for the date is the day I resign from old job or the day that Cobra begins. I wish to keep Cobra longer because it has benefits the new insurance doesn't that I wish to use and I also met my OOP. I know it will cost me but it will still be beneficial. Here are the details:
— I leave old job on August 15th.
— I start new job on August 16th.
— New job insurance can kick in on August 21st.
— Old job insurance ends on August 31st, so Cobra can be effective as of Sep 1st.
Given that new insurance will have started while I still have the old insurance and before Cobra, will I be able to keep Cobra? Will that be the date they consider or will they look at the end date of the old job? Thank you all for the information you might share, its for important medical procedures and I'm trying to figure this out!

submitted by /u/Aware-Passenger7115
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