If you're welding the two cars together connecting the axles you're still having 8 contact patches instead of 4, so the axle load is the same as 2 separate vehicles they're just moving in tandem.

You'd need to stack the two cars on top of each other to increase the axle load. In which case I'd say it's not obvious how much more the road wear would be without looking at data.

I'm not saying the 4th power law is absolute truth, I truly don't know what the wear patterns would look like on a modern surface. But your example isn't proving it wrong at all.