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.

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.

Doug, Your comments are a perfect rationale for why I have become a devotee of idlers and direct-drives. You make a good point about the trade-off associated with having the motor pulley close as possible to the platter, but are we conflating belt "creep" with belt "slip"? I had not thought of that (mostly because I only worry about belt creep after the third slice of pizza). I was actually repeating a statement once made by Mark Kelly, or at least I thought I was. If the idea is flawed, then I probably misinterpreted Mark. There are two better solutions to belt creep. One is using a capstan-like device to keep the belt close to BOTH the pulley and the platter. (One commercial turntable does that, I think.) The other is to use two platters, one driving the other, with the belt wrapped in a way described by Mark and maybe also implemented by RS Labs, such that it is in contact with most of the circumference of both the driver and the driven platters.

I can only imagine one reason why the motors must be equidistant. First, it would seem obvious that we want the two motors to be as identical as possible in all parameters. If so, then it follows that we want their drive pulleys to be rotating at identical speeds, or as identical as possible. Therefore the belts need to be of the identical circumference. From that, does it not follow they would be equidistant from the spindle?

Third try. My last two posts never got posted. In the above post, I misspoke; the 2-platter solution to belt creep is commercially implemented in the 47 Labs turntable, not the RS Labs one. (In fact, RS Labs does not make a turntable, so far as I can tell.)

John, I owned a Hyperspace. Great turntable. As I recall, David Fletcher's crude instruction sheet advised placing the edge of the pulley practically touching the edge of the platter, a few mm's apart. When I later read about belt creep, I understood better why he did so. A Walker Audio Motor Controller made a profound improvement in the sound of my Hyperspace, much to my amazement.