Any reason why the top 2? With just the top, you wouldn't need an extra round and the competitors wouldn't need to mix the same sources twice.
For the mixing, it could be determined by taking the 3 competitors in a match, then flipping a coin to decide whether to take the 2nd source from above or below.
For example:
FLIP - Heads (above)
ladyWildfire - 1st: Aqua Planet, 2nd: Oil Ocean
Syllix - 1st: Angel Island, 2nd: Aqua Planet
halc - 1st: Oil Ocean, 2nd: Angel Island