If the balances are exactly as in the example, AFAIK the previous posters are correct. If they're a bit more skewed, several airlines (AA is one example) sell their award tickets on a one-way basis. You can get three of the one-ways from one account, for 3/4 of the total miles, and the fourth for 1/4 of the miles from the other.
With AA, at least, there's no penalty for doing this. They don't even offer round-trip awards. I think there are airlines that offer both, with a one-way award requiring more than half the miles of a round trip. In that case this isn't a desirable strategy, but it may still be worth it if buying or transferring the miles would cost a lot.