Was the P-82 twin Mustang better than the P-51D?
The two planes serve very different roles. The 82 was a long distance aircraft first and foremost, and everything else was secondary to that goal. The 51 was a fighter, and its excellent range was not a specific design criterion, but the result of several unrelated design decisions that worked in happy concert.
So, based on that alone, I think the answer is clear.
But the real issue with this comparison is the time dimension, the 82 saw combat when it was already outdated, whereas the 51 saw combat when it was the pinnacle of performance. Jets were already the hotness when the 82 was on the front line, and I doubt anyone looked favorably on being assigned to them when the alternative was an 84 or 86.