You cannot transfer two firearms on different dates by using the 4473. It's why ffl's would have you fill out two 4473's on the same day if you were buying an 870 and an m&p. They'd submit the 77r1,2 &3 to the state police along with the 4473 with the m&p on it...and the they'd take the 4473 with the 870 on it and call in an fbi nics check. They'd write the NTN on that 4473(with the 870 and the box for "long gun" checked). If the check came back with a proceed, the representative from the ffl would then sign and date the third page of the 4473 to record when the transfer took place.
The 4473 with the regulated firearm would still need it's own ntn and the ffl rep would need to sign and date when that transfer took place.
THIS