A NFL record? Yes, according to AP.

In an AP article on the Brown's-Bengals game, the reporter wrote that it was the 3rd time in NFL history that 2 QBs threw 5 TDs, each.

The first occurred in 1963 by Tom Flores (Oakland) and George Blanda (Houston).

In 1963 both Oakland and Houston were in the AFL and there was no association between the 2 leagues.

Upon the merger, did both leagues also agree to merge their already existing record books?

 

I believe so. I think they also did the same thing with the AAFL in the 1940s, but not the USFL for the same reason.

 

yep, the NFL absorbed all records, stats and history of the AFL when they merged

it's actually the fourth time it's happened though

 

They did, and that would make sense because the NFL accepted all of the AFL teams. The biggest difference in rules was that in the AFL, you could go for a two-point conversion, so the rules weren't so different that the game wasn't comparable.