How far out to place motor from platter

It's your call. For minimum belt creep, the platter and motor pulley should be as close as possible, so that the circumference of the platter is in contact with as much of the belt as is possible. For other reasons, you might want them to be far apart.

note that your speed may change based on how tight the belt is, either due to belt stretch changing the diameter or excess loading on the motor bearing increasing friction. Obviously, both motors should be the same distance away from the platter.

Manitunc, OP asked about distance. Good point to say that both motors should be the equidistant from the spindle. Along those lines, and altho the OP did not ask, the best way to use two motors would be to line them up on opposite sides of the platter. In other words, 180 degrees apart. That way, the belt or strings are "pulling" on the platter in opposite directions, and those side forces should cancel, assuming tension is equal on both strings. That's the only good thing about using more than one motor, IMO.