See Japan (2) and the US (3).

DC links are now feasible for these situations, and don't have certain failure properties of AC grids (synchronization/frequency/phase).

Now they're one grid with a controllable weak link. They might solve the all-or-nothing interconnection problem of AC grids, but they can still cascade failures - and they will, if one goes down and everyone switches to the other.

The two Japanese systems have 130+ GW of generating capacity each and the links between them have a total capacity of about 2 GW (<1%).