The 2d4 have a very different distribution curve to the 1d8.
Between the pair of them the 2d4 have an almost 50% chance of one of them acing, so you might expect the average to be 2½ + 2½ + ½x2½ = 6¾ whereas the d8 will average 4½ with only a 1 in 8 chance of acing. But the the d8 will, 1 in 8 times, give a total bigger than 8, and the chances of getting a 15 are the same as getting a 9. The 2d4 really struggle to get 15.