These don't necessarily have to be synched to absolute perfection. When I was doing the 70mm mix I found that Belbucus' files tended to be off by anywhere from 10 to 52 samples from each other. This is a very impressively small margin of error, certainly not even remotely noticeable--the largest discrepancy is only just over a millisecond, far less than one frame. Trust me, nobody's going to notice that at all. If they're going off by large amounts, then something is seriously wrong, because the work on those is all done already.