Neither hurricane posed an immediate threat to land.
It was expected to strengthen over the next two days as it heads west-northwest, well away from land.
The hurricane center projected the storm could take a sudden eastward turn early next week, putting it on a path toward Mexico's southern Pacific coast.
That prediction is tentative and depends on a tropical wave of pressure forming in the western Caribbean, said John Cangialosi, a hurricane specialist at the center. Otherwise, Darby could continue on its northwestern path out to sea.
"It's a low-confidence forecast at this point," Cangialosi said. "It's not going to have great impact on land over next few days so there is certainly time to watch it."
Meanwhile, Celia became a Category 4 hurricane as it headed west across the open Pacific. Its maximum sustained winds increased to 135 mph.
It was located about 765 miles off the southern tip of Mexico's Baja California peninsula.