That is the dealer's responsibility to have you fill out that form. The requirement is 5 consecutive business days. So, you buy one on Monday and buy the second one next Monday. Nowadays a business day can be a Sat or Sunday. Who knows.
As most say, don't worry about it. A woman up here in Maine bought 52 guns in a 6 month period. How did they catch her? One of the guns was used in a murder in Calif.
Otherwise they probably wouldn't have known she bought all those guns. Oh, 38 were from the same dealer, so basically she had to fill out one of those forms at least once. She's in prison right now for straw purchases.
BTW, when you do a 4473 "background check" the serial # of the firearm you purchase is on that form. Although it stays with the dealer, that's how the Feds track it back to the person that purchased it.