Since they're going at the same speed and at a 60 degree angle, the third side of the triangle they make is also 1200. Dividing 1200 miles by 500 mph gives 2.4 hours.
Or, you could split the triangle in half and use the sine of 30 degrees (0.5) to find half the length of the distance between the two (600), and then double that to get 1200.