Early P-38s had the counter rotating engines. They were mounted opposite what it is today.
They found it couldn’t fly off the ground in the required length of runway. In the course of, What are we gonna do about, discussions someone suggested switching engine positions.
They did that and it improved its takeoff performance.

Is there some sort of fence on the wingtip?

Counter rotating outwards for different reasons. The Aerostar has it to reduce slip stream drag on the fuselage. The P38 had its rotation changed from inward to outwards because it allegedly made the gun platform more stable.

Increasing horsepower can make the airplane less stable in pitch. The original 601P Aerostar wasn’t very longitudinally stable to start with, and the stick force per G slope was very low. When Machen installed 350 hp engines we tested the airplane to see if it would pass the longitudinal stability test and it didn’t. The solution was to add a stability bob weight to the elevator control system. This made the airplane way more stable and also increased the stick force per G.
When Piper tried to certify their 700P they reversed the counter rotating engine to contra-rotating where the down turning blade was outside of the nacelles in an effort to improve the longitudinal stability. That wasn’t enough so they closed up the aft CG limit. That didn’t fix the problem either, so they copied the bob weight, that Machen developed and that fixed it. Why didn’t they reverse the engines back to the original configuration? I don’t know the answer but I did hear that the test pilots thought it was quieter. I suspect it was time to certify the airplane and they just said certify the airplane as is. I have flown several 700P’s, conventional right hand rotating engine Aerostars, and the counter rotating versions. There is no significant difference in noise but the counter rotating airplane climbs better with the left engine inoperative than with the right engine rotating right.

The was the story I've read many times about the XP-82 Twin Mustang, but I hadn't heard that about the P-38 before.

My info came from The late Tony Levier, who was a Lockheed Test Pilot during WWII.
According to him, changing engine location allowed the P-38 to have a shorter ground roll.

No disrespect to Tony but he never flew the XP-38, the only one with inward props, the change was made to outward rotation before he joined the program. The Aircraft Engine Historical Society info that it was for stability and buffeting comes from P-38 designer Kelly Johnson.

I understood, in the case of the P-38, it had to do with prop wash on the tail, but not sure if it was the vertical or horizontal stab.