It really does read that […] only 1 segment will earn the bonus status credits.
This offer cannot be used in conjunction with any other offer by Qantas. Only one Double Status Credits offer can be used per individual qualifying segment of your itinerary. If you qualify for multiple bonus offers, the offer first registered for will be applied to each individual qualifying segment.
example would be. MEL-SYD-BNE in business class. According to that, they will only allow the bonus credits on the MEL-SYD […].
Similar text appeared on previous DSC offers. It says one offer per segment, not one segment per offer. So I'm sure that all qualifying segments in an itinerary would get the double SCs. In the unlikely event there were two different Qantas offers in play at once (unclear how this would happen when the booking windows are only a few days!), this text would tell you which offer would apply to each of your itineraries' segments.