You can connect two running grids. Earlier this year the Baltic countries disconnected from the Russian grid, and synchronized and then connected to the European grid.

I imagine you can get close enough by syncing to a shared time source like GPS or the DCF77 signal, as long as you communicate how the phase is supposed to match up to the time source. Or at least you could get close enough that you can then quickly sync the islands the traditional way.

The question is if it's worth the effort and risk. Cold starting a power grid is a once in a lifetime event (at least in Europe, I imagine some grids are less stable) and Spain seems to plan to have everything back up again in 10 hours. Maybe if the entire European grid went down we would attempt something like that by having each country start up on their own, then synchronize and reconnect the European grid over the following week.

https://en.wikipedia.org/wiki/Baltic_states_synchronization_...