The advice here is correct. The only thing not mentioned is whether or not you have a CPL...
In both cases, transfer both lowers as "Other" on the 4473. Build one into a rifle when time permits, and you're done. No extra paperwork.
For the lower that will become a pistol:
If you have a CPL, you will build the lower into a pistol when time permits, and within 10 days of completing it, you will drop off the RI-60 with you as buyer (using your CPL number) and seller at your local LE station.
If you don't have a CPL, you will go to your local LE station and obtain a pistol purchase permit which is valid for 30 days. During those 30 days, you will assemble the lower into a pistol. Within 10 days of completion of the pistol, you will submit the RI-60 with you as buyer (using the NICS check number from the pistol purchase permit) and seller to your local LE station.