This research is a result of a debate in the "Playing the Game" forum over the merits of Earthquake verus Bonemerang. Here's what I found...
Let's take a look at a max stat Marowak attacking a max stat Mewtwo with Earthquake and Bonemerang.
Earthquake: ((100 * 258 * 100) / (128 * 278) + 1) * 1.5 = 110 HP
Bonemerang: ((50 * 258 * 100) / (128 * 278) + 1) * 1.5 = 55 HP * 2 = 110 HP
Now, this is based on the assumption that the game will always round the values down to the next whole number (which I am almost 100% certain of, since that's the way computers normally deal with integers) and that this rounding is done after each hit (i.e. it's done twice for Bonemerang). However, if it rounds off to the nearest whole number, the final value could be as high as 112 HP for Bonemerang. So let's go with that for now. When we factor in accuracy to get the average damage:
Earthquake: 110 * 255/256 = 109.57 HP
Bonemerang: 112 * 229/256 = 100.19 HP
So, it still does less damage. To see how much difference this can make at extreme levels, let's pretend that Mewtwo has a double weakness to Ground attacks:
Earthquake: ((100 * 258 * 100) / (128 * 278) + 1) * 1.5 * 4 = 441 HP
Bonemerang: ((50 * 258 * 100) / (128 * 278) + 1) * 1.5 * 4 = 223 HP * 2 = 446 HP
Again, if I assume that it would round up instead, the Bonemerang damage would be 448 HP instead. Factor in accuracy:
Earthquake: 441 * 255/256 = 439.28 HP
Bonemerang: 448 * 229/256 = 400.75 HP
The average damage is still considerably less with Bonemerang (about 91% of what Earthquake gets). The only situation I can see Bonemerang being better in is if the couple extra HP allows it to finish off an opponent, or break a substitute, or do slightly more than 50% damage against a Pokemon with Recover/Softboiled.
The only example I can find of this giving a multi-hit attack a definite advantage over a single-hit one of the same total power is Double Kick vs. Rolling Kick (although the latter can paralyze) or Bite/Swift (unless type modifiers affect them).