The point about transferring points across accounts has been made above. HUge cost and not worth it

But assume that you have enough points to do either strategy. Then the question is availability.
Finding 2J awards for SYD-EUR is going to be rare.
If you first book one award, you cannot be certain that yield management will keep the other J award open. It probably will, but never certain. Or the other award could be picked up while you are going through the process of selecting the first award.