Going back to the issue of clock drift between two outboard, mono recorders: The Lectrosonics recorders are specified to have better than 1 ppm clock accuracy by means of a temperature-controlled crystal oscillator ("TCXO") circuit. Tascam doesn't give any such specification for the DR-10 recorders, but the drift I observed between my two was nearly twice the maximum allowable difference between any two Lectrosonics recorders.
That's not apples vs. apples, though. In terms of clock rate variation, I don't know how typical my two Tascam recorders are of their species. Plus if these errors are randomly distributed, the discrepancy between the clock rates of two randomly selected Lectrosonics recorders should, in the great majority of cases, be distinctly less than the maximum allowable difference.
It's interesting to realize the degree to which even now, some 40 years into the digital recording "revolution", the clocks in professional recorders still can't stay in sync with one another even for modest periods of time. 1 ppm accuracy, if I'm figuring this correctly, means a potential deviation of about three samples per minute at 48 kHz--so two separate recorders each having 1 ppm accuracy could slip apart at the rate of one sample every ten seconds or so.
Of course, thinking back, the very best analog studio recorders (with crystal-controlled servo capstan motors and advanced tape tension controls on both the supply and take-up reels) specified 0.1% speed accuracy, a/k/a 1,000 ppm ... so maybe I shouldn't complain too loudly.
--best regards