The problem with trying to have two grids is that if anyone ever connects them together, now they're one grid with a weak link in it.

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).

	▲	cowlevel a day ago \| parent \| next [-]
		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.
	▲	formerly_proven 18 hours ago \| parent \| prev [-]
		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%).