bear in mind the total allowed mileage purchase (before bonuses) is 100k per account per year. And it's probably a good idea to work out how to max any bonus allowed within a promotion (ie lower mileage purchases may not attract the max bonus miles, and any miles you have left within your limit for next time may also then not attract the max bonus).
If you have one account for each passenger, max out just on one account, and buy both tickets from that (which is what I think you might have had in mind anyway).
Great tip! I was thinking along the same lines. One account has approx 50K points from the US transfer - so if i go for the 100 + 60 option, I should have enough for 2x one way Europe-AUS flights and be left with 3x internal AU/AU-NZ flights.