Reducing long term unpredictability in pairs of crystal oscillators?
For long term relative synchronicity you are better overall to find a way to link up each system to a reference on a periodic basis to null out long term drift. There will be a tradeoff in how often you do this versus the power consumed to do so and the money spent on circuit performance/complexity to achieve longer term uncoordinated sync.

Consideration should also be made toward realizing that matched components are never going to be perfect. As such you have to look toward higher level techniques to calibrate and null out undesirable characteristics.