You'd get the same phenomenon that you see when operating turn signals in traffic. They seem to weave "in and out" of sync. The frequency at which that happens is the beat frequency, i.e. the difference between the two blinking frequencies.
It's a bit larger than a 555, but it should keep you within a small handful of microseconds per day until the aging effects start to add up which make getting more than a millisecond a year dicey, even if the thing doesn't die on you: https://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resource...
If you have shared line power you can just use that and everything will be locked in sync forever.
If you don't want to use that or radio, and you are outside, you could try to be really clever am sync your flash phases to a specific position of the sun. This is what the Long Now clock does. It'll be a different time each day, but it'll be the same for all units, within a small tolerance.
I mean, sure, the TCXO will mean that you only start seeing a phase difference between the two after weeks instead of minutes, but what's the point of that? I you want them to be at the same phase, you'll need to sync them at some point, and you do that by using a common clock source.
So either you shell out some effort for a real solution (power line is nice, and also qualifies as a common clock source as I've predicated), or you don't. And if you don't, there's no point in using a precise-ish clock at all, and you'll likely end up with very quick desyncing.
But presumably these lights at least have battery backup, given the obvious risks in case all of them were to fail at the same time due to a grid issue.
(Doesn't solve the problem if you want them to be in sync phase-wise, i.e. blink at the same time or similar, but at least they won't drift apart, which was what this is about.)
For example, say you have a scheme where a period longer than the last one is symbol A, about the same period is B and shorter is C. You will get a random-ish sequence of symbols.
If you have an algorithm that, say, resets the timer to zero whenever a certain symbol sequence is detected, you can eventually get back in sync. With some care you can make sure you only sync when the sequence happens and the light has only been off for a short period to avoid excessively long off periods or truncated on periods.
Then you just need to have a local oscillator good enough to do that timing analysis and that can maintain sync between these symbol occurrences.
You could do it on the tiniest micro. Once you've counted the zero crossing detector, these days you might save 3 to 5 whole dollars over a GPS receiver on your very expensive ICAO compliant lamp and also ruled out using DC into the bargain! And theoretically it desyncs when the grid is too stable for days on end (and you just get BBBBB or ABABAB for millions of cycles)!
In terms of what is actually used, they do often use GPS and many of them have MODBUS or similar data connections which presumably wire into the wind turbine's telemetry somehow for fault detection.
