brushless 1/5 4wd conversion about to begin! couple questions.

[RC-Monster Mike]

It is generally considered bad practice to use set screws at 180 degrees from each other on a shaft in applications like this. This would be less secure than a single set screw. The very tip of both set screws becomes the only contact points on the shaft, while a single set screw allows the entire opposite side of the bore to engage the shaft - much larger surface area for securing the gear.
If 2 set screws are desired, they should be located 90 degrees apart. :)