It turns out that if you want two NBN connections (assuming you can even get one) there's no way to do it - even if you're prepared to pay for it. If you have a second building on your property, like a granny flat or home office that want to connect separately, there's no way to do it according to NBN Co's installation rules.
There's an entire political argument going on about this with the Australian Greens' NBN spokesperson and the communications minister sending letters to each other - I guess that's the Senate equivalent of passing "you're not my friend anymore" notes in class - but I don't want to get into that. You can read about the specific rules here.
Back when I built my own home office, I wanted to have that space configured with its own connection so that I could keep my home and internet connections separate. I figured, at the time, that doing that would make things much neater from an expense tracking and tax point of view.
Given the high cost of housing today, it's easy to imagine people adding secondary dwellings on their property either to house other family members - the good old "granny flat" or setting up a small residence that can be rented out. And, it would make sense to have those buildings established with their own infrastructure as much as possible.
When I built my office, I ran an Ethernet cable (in the appropriate conduits that were supplied and installed by a qualified professional) from the house to the office so that I could share the connection. Back then, I could not convince my current ISP at the time, nor any other, to install a second internet connection. And that was even though I had both Optus and Bigpond cable already drawn from the pole to the property.
Once I had the cable installed, it was a matter of setting up a second router in the office (with routing and NAT disabled) so that I could use the internet connection from the house.
For those running a second residence on their property, it seems that they only solution is to either ensure you can get a solid wireless signal to the second building - a weatherproof access point might do the trick - or run cable from the connection point to the second building. Then you need to agree on how to share the costs fairly if that's an issue.
Does the "one connection per address" rule affect you? How would you work around it?