Hi Canberran,
Can't see who wouldn't insure you if you have unit titled the property. They are considered two different properties (like a strata title in other states) with two different titles and you could insure them separately with two different insurers if you feel like it (don't know why you would though). Anyway, a lot of companies banks, insurers etc outside of Canberra do not understand ACT terms, so you might want to call it dual occupancy and you shouldn't have any troubles.
