This is confusing a retail customer brokerage account with how naked short selling occurrs
There a multiple ways this is accomplished. One easy to follow way using the FTD scenario with MMs:
MM#1 naked short sells 2,000,000 shs ELTP a few days later receives a call to deliver MM#2 shorts 2,000,000 shs ELTP to MM#1, covering MM#1 deliver requirement MM#1 is now flat MM#2 maintains the naked short of 2,000,000 shs rinse and repeat