building:part doesn’t mean that there is an additional / another part to a building, it merely describes it in detail. If your building:part covers the area of your building, then the renderers will only render the building:part. Only where there are no building:parts, renderers will fall back to rendering those like the building. So if you want to go for this approach, you will need 1 building with 2 overlapping building:parts.
But this is completely overcomplicating things. I’d go with this simple approach:
building=roof
height=7 m
min_height=5 m
roof:height=1 m
roof:shape=hipped
roof:orientation=along/across
layer=1