Sometimes 2 one-way tickets are cheaper, it varies by route (for example SYD-AKL-SYD is always cheaper when bought separately, at least for J fares). Most web sites don't allow specifying different class on different sectors, but a few do. Easiest way is to use a travel agent and for mixed class bookings they usually have access to lower fares (typically 5-10% lower). If you have an Amex card there could be other benefits in using Amex travel to buy tickets, offseting their fee.