Hi there,
I would definitely suggest providing an explanatory letter re finances, we (NZ girl and French guy) provided a document explaining our financial situation and referring the person reviewing our application to various aspects of our individual and separate accounts that we wanted them to be aware of. For example, our separate accounts might have shown we were both in Stewart Island at the same bar on the same night (each paying for drinks), i.e. things that showed we were constantly together (we normally reside in Queenstown), we highlighted the transactions in our account joint and separate accounts and then provided an explanations about them etc. If your financial situation is a bit different, ours was, my Auckland based parents own the house we live in in Queenstown, there's no mortgage and we are lucky that they don't ask us to pay rent - provide an explanation. We explained why there were no tenancy agreements and why there were no rental/mortgage payments going out of our accounts. We'd only been living together 3 months and were granted a partnership visa.
Further, 2 years is tricky to get straight off the mark. We were given 6, then after that 12 months. We've been told that we're pretty much guaranteed 2 years for the next application but we've been living together nearly 2 years now.
Hopefully that helps, let me know if you have any more questions about it.
Cheers,