I think it would be a problem if they had ten tiny grids, but is two decently sized grids that are in roughly the same time zone (so peak solar doesn’t need to go from A to B) a big deal?

It is when a 1500MW nuclear power plant built in area A is essentially useless for anyone in area B.

Yes, you can transform from 50Hz to 60Hz, but the capacity is limited and can't be grown infinitely.

It was a huge problem when earthquake stopped many power plants in east area.