I have a requirement where I need two Suns (both Netra T1s) to have
their clocks close to each other during an experiment that lasts 4
hours. During this time, neither Sun will have internet access and they
will not be networked to each other.

I suspect if I sync them by NTP before the experiment starts, then they
will not drift by more than 500 ms over a 4 hour period. Does that seem
reasonable? Has anyone actually ever made medium term (few hours)
stability measurements of the clocks in Suns?