OK ! Firstly thankyou one and all for all the very informative replies. 
I am pleased to recieve so many replies to this question because firstly, as a typical homeowner I am certainly no expert in this field and trying to find out exactly what each homeowner should and must do wasn’t very easy either.
Thank you scotlandx because there were several replies with I need to register something with someone but you have stated: “…you need to resolve to pass by-laws and register those by-laws with the Department of Lands….”
Thank you for making that clear and that is what I had been hoping for. A source for where I can ask the right questions and get the right answers.
You also state: “…If you want to make each owner responsible for specific parts of the property, then you need to determine what they are, and that needs to be spelt out in the by-laws, because what you are doing or would like to do is giving each other exclusive use…”
Thank you again for your clarity.
kiwipaul, thankyou for your continued persistence to make me see the reality of things legally rather than a forelorn hope until something sours.
IBC, our duplex is a registered strata with a strata plan number and I do thank you for your diplomatic reply and strong suggestion of further legal advice.
So I have searched the internet and found this site: https://www.lands.nsw.gov.au/
The Land and Property Management Authority (LPMA) was abolished under the NSW Government restructure announced in April 2011 and its former business divisions transferred to new departments. All divisions are continuing business as usual and the previous customer contact points are available.
I have made a call to the office Land and Property information:
They have confirmed that the strata plan for this property as registered with them have adopted the model strata by laws
I have made a call and being friday afternoon, they have advised a senior person will call me back Monday in regards to:
1) a duplex situation under a strata plan and what needs to be registered
2) what needs to be done to make both owners responsible for their own homes in terms of any repairs or capital improvements.
I do appreciate all the advice provided as it has proved to be invaluable and has highlighted my own shortfalls and lack of knowledge and expertise and the pitfalls in buying into a duplex strata, however, we were just lucky that so far there has been zero issues and no fallout between the 2 parties.
Of course this could change and everything could go sour and that is what we both want to protect against.
We didn’t really want to explore the Torrens title option as that will involve additional expense, but in the end if we need to consult our solicitors/ lawyers then it will be at a cost anyway to both parties, then perhaps a Torrens application might be a better outcome.
I will post a reply when I have spoken to a representative from the Land and Property information office.
I had sent an email to NSW fair Trading but they failed to reply within their 24 hr timeframe, so maybe Monday.
This certainly isn’t as easy as I first thought and I do appreciate this forum for providing an avenue and link to yourselves and your invaluable insight and advice 
Thank you one and all so far and keep the advice coming with whatever you have to share that can shed more light on what is a complex but interesting exercise for me.
I now wonder…When I can feel secure and I can just hold yearly coffee catch ups and have a chat..