A cheapo 20ppm quartz watch crystal definitely doesn't drift by a second every few minutes, but it will over about about half a day at maximum error. A 5ppm one (still sub one Euro) will keep a second about every two days. A 0.5ppm TCXO can be had for about 15 Euros from Mouser and that gets you two weeks.
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.
Well, yeah. I was sort of assuming that you won't use a TCXO or even just a cheapo quartz crystal, because it doesn't make a lot of sense: You've now thrown money at something that will relatively quickly desync anyway.
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.
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.