Advantages of splitting a single NMOS transistor into two
It comes down to several aspects of transistor design (implants, process , manufacturing sequence etc.) and how the transistor characteristics change with size.


Threshold is not constant with gate length

for "analog" transistors threshold voltage shifts upwards with length
For "pocket" or Halo implanted transistors the threshold peaks at the minimum gate length and decreases for longer gate lengths

For layout reasons:

often for matching reasons it is best to place multiple shorter length transistors in a grid and then wire them up as you see fit, rather then just making one big one,
this also increases yield as the pattern density on die does not change too much

another alternative is that the designer maybe be playing with back gate effects (AKA bulk) connection and how that modulates the threshold.  Particularly in a PMOS where the NWell is not tied to the substrate.