U.S. law requires two inlets for anti entrapment purposes.
The pump will pull whatever its capacity is, regardless of the number of inlets. Now, going from two to one would increase the velocity of the water moving into the inlet, which may be a slight benefit. But that pump/filter unit is only slightly better than nothing, so I'm not sure I'd go to the trouble.
We had that exact same pool several years back in our pool "evolution" (that has coincided with our daughter aging). That was the pool where I finally decided it was too big to dump the water every time it turned green, and decided to figure out how to prevent it from happening. Coincidentally, that was when I found TFP.! I got a good test kit, bought chlorine, threw away the pucks, the algaecides, the clarifiers, and all the other snake oil, and learned how to test and maintain our pool. It's a skill that's served me well as we are now at our current 16' x 48" pool.
Even though it's later in the year, why not take the opportunity to "perfect your craft?" Hone your skills now prior to closing up the pool for the season, and you'll hit the ground running next year!