I've just looked at my notes from years ago, ehich ar esketchy and
incomplete, but they tell me that Keller was using separate cutter heads
cutting two separate grooves on a wax master, to be played back using
separate cartridges, a la Emory Cook; Maxfield's patent suggests that he
was anticipating recording using two synchronized lathes, powered via
gearing from a single motor. RMI did patent Blumlein's 45/45 stereo
disc, but according to the bio of Blumlein I read the patent ran out in
about 1950. EMI unaccountably failed to renew it, leaving Western
Electric a clear field when they rediscovered the technique.
Unfortunately I don't have my records of all the sources for this info.