There's plenty of all day events that do just in fine in Australia. Test cricket, formula 1, several large horse racing events, Australian Open tennis, many music festivals. You might be right about most of these being once in a year events, but so is the 7's and so would be a double header.
The perceived importance or relevance of an event is the primary driver of crowd numbers in my opinion.
Test cricket gets roughly double the crowd sizes to go to the Ashes than anything else in Australia.
Music festivals attract audiences relative to the size of the biggest acts playing. If you want 50,000 people at a festival, your headliners need to be acts that fill arenas.
I think an NRC double header would attract more people than for a single game, but I think the overall crowd at this stage is unlikely to be more than double the crowd you would have got at a single game.