"For 2020 there were no RMDs, so this strategy works for that but not for most years."

As long as there was not a Conversion. The ability to make a Redeposit of the RMD as if that is a rollover requires that to be the same Nontaxable event as any other Rollover.

"Realized he could covert to ROTH instead of taking RMD."

That was never changed, and that is not changed or covered by CARES.

"RMD was shipped to his primary (major) money management firm."

Put into Investment account = he took the distribution.

"This account (Primary)"

It's more like a Customer Profile. You still work by Actual Account. They are not combined or intertwined. You cannot have a Joint IRA, for example, but that MM is likely Joint.

"and then, same day, transferred 24K into ROTH IRA."

Not CARES related.

"He received 2 Forms 1099-R. The first (24K) was from the original Traditional IRA Account in the first firm."

You show that as a regular rollover. You have to address any withholding, though. Was the RMD rolled in Gross or Net?

"He then received a second Form 1099-R from his primary account for 24K"

There is no "Primary" account. There is that Reporting Entity.

"for the same amount of money. He thinks he is double-taxed."

He hasn't been Taxed. That's why I asked about Withholding. He prepaid. That isn't Tax, yet.

"Any ideas on how to document this? (I am afraid I know the answer.)"

You need to address the amount distributed vs rolled/conversion.