[Nonfractal]

Any reason not to use 2 screws?

There might be a very good reason not to use 2 set screws in a slot car wheel.
I've been looking up "bearing location"  in larger scale engineering. 
Most bearings have only one set screw and are often located with a corresponding flat spot on the shaft. 

Some bearings use two, but these are never 180 degrees separated
90 degree or sometimes 120 degree is used.

This must be related to the pressure between the shaft and the inner face of the bearning. 
Maybe set screws at 180 degrees apart would work against  each other. 

If you use two screws at 180 degrees apart, maybe if one comes loose, the other does not compensate, so maybe you're doubling the chance of failure instead of halving it...

AlanW