How much would an additional motor help

Are the two motors positioned 180 degrees from each other and the same distance from the platter or are they in some other configuration?

There are plenty of theoretical reasons why extra motors is a bad idea, but, since you have an opportunity to hear the result, don't worry at all about them. I have no idea if that difference alone has a positive or negative result.

What I do know is that I heard great sound - very dynamic and incisive sound from a three motor machine: the big table from Audionote. It had three huge power supplies for the motors that made it impractical for my system so I passed on the opportunity to own one at a decent price.