Hopefully this will explain. I can book eco on both legs on Emirates, and then upgrade one of those (incoming) to business. In order to get business on one leg on Qantas, I would've had to book eco+ on both legs, then upgrade.

I realise now while typing it that this may not be the case, as the only time I've ever tried to upgrade before this was on BA when I was told explicitly that you cannot upgrade by more than 1 class change on a booking, so eco to eco+ only (on 4 class flights), eco+ to bus only, etc etc. I don't actually know if this is the policy with all airlines.