This is what the Strata Act says about two-lot plans:
69 Sinking fund to be established
(1) An owners corporation must establish a sinking fund.
(2) However, an owners corporation for a strata scheme comprising 2 lots need not establish a sinking fund if:
(a) the owners corporation so determines by unanimous resolution, and
(b) the buildings comprised in one of those lots are physically detached from the buildings comprised in the other lot, and
(c) no building or part of a building in the strata scheme is situated outside those lots.
So it sounds like you DO need a sinking fund. If that’s the case, then this applies:
75 Estimates to be prepared of contributions to administrative and sinking funds
(1) An owners corporation must, not later than 14 days after the constitution of the owners corporation and at each annual general meeting after that, estimate how much money it will need to credit to its administrative fund for actual and expected expenditure:
(a) to maintain in good condition on a day-to-day basis the common property and any personal property vested in the owners corporation, and
(b) to provide for insurance premiums, and
(c) to meet other recurrent expenses.
Note. Recurrent expenses would include such regular expenses as insurance, water charges, electricity charges, carpet cleaning, lawnmowing services and the like and minor expenses relating to maintenance of the common property.
(2) An owners corporation must, at each annual general meeting, estimate how much money it will need to credit to its sinking fund for actual and expected expenditure:
(a) for painting or repainting any part of the common property which is a building or other structure, and
(b) to acquire personal property, and
(c) to renew or replace personal property, and
(d) to renew or replace fixtures and fittings that are part of the common property, and
(e) to replace or repair the common property, and
(f) to meet other expenses of a capital nature.
Note. Expenses of a capital nature would include expenses in relation to major repairs or improvements to the common property or personal property of the owners corporation, such as painting of a building or replacement of roofing, guttering or fences and the like.
(3) When estimating amounts needed to be credited to the administrative fund or the sinking fund the owners corporation must have before it, and take into account, a statement of the existing financial situation of the strata scheme and an estimate of receipts and payments.
(4) In estimating amounts to be credited to the sinking fund, an owners corporation that is required to prepare a plan under section 75A is to take into account anticipated major expenditure identified in the plan for the 10-year period to which the plan relates.
Professional surveyors can make the sinking fund assessment for you but in a small strata you might be able to do so yourself with the assistance of local tradesmen who ill tell you when maintenance will be required and how much it will cost.
It would make sense for you to have a separate joint account, perhaps accessible only by signature of both parties, to show prospective purchasers that everything is in hand and above board and the required funds are in place. This shouldn’t cost any more than the time it takes to set it up