There are “split head” mattresses where only the top section of the mattress is split. People may also call these “flex top” or “half-split” beds. Split head/flex top beds are very rare in comparison to a standard split king mattress. Most of the time, you’ll only find these through specialty sellers or some brick and mortar stores, and they often don’t have many reviews compared to other mattresses on the market.
Flex top beds are also used with adjustable mattresses to allow sleepers to raise or lower their heads independently.