Its not like low light vision.
If you have 60' Darkvision, low light-vision, and a lit torch in a cave, you'd be able to see 40 feet normally, and the last 20 feet is dark vision.
If you have 60' Darkvision, NO low light-vision, and a lit torch in a cave, you would be able to see 20 feet normally, and the last 40 feet in dark vision.
And if their was no light, a cloudy, moonless night, in a cave, or what have you. You could see 60 feet.
Just remember, darkvision is black and white only.