If these are two separate houses in one building, there's ways to tag this correctly instead of making invalid multipolygons. You can tag them as building=house and house=semi-detatched, or building=semidetatched_house, probably worth checking to see which option is more widely used in Australia.

If you're not sure whether it's a semidetatched house or just one house, it should just be a single polygon and building=house, can always add a fixme to confirm which way it is.
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            Published using OSMCha: https://osmcha.org/changesets/158386943
            