from reading through @curlicuecal's old bug xeno troll headcanons, I started to think, "what if a species that uses another species as part of its reproductive lifecycle, but they're both sapient?"
and what i came up with was: a world that's mostly covered in water, dotted with small volcanic islands that can only support a very small number of organisms as large as humans. the land-dwelling species crawled out of the ocean to take advantage of the much more abundant/concentrated resources on land, but as they tried to spread to smaller and smaller islands, groups frequently died out due to population bottlenecks and lack of genetic diversity.
enter the second species, who are sea-dwelling. there's much fiercer competition for resources in the sea, so they started hanging around landdweller-occupied islands cause they'd sometimes, e.g. drop fruit into the water.
what eventually developed was a symbiosis in which landdwellers provide seadwellers with nutrients in exchange for seadwellers spreading landdwellers' genes to distant islands. in the species' modern form, the seadwellers have a dedicated body cavity for receiving landdwellers' sexual emissions and extracting nutrients from it while preserving the genetic material for future matings. landdwellers are fully hermaphroditic, they can't afford sexual specialization when chances to mate outside their very small local gene pool are so rare and valuable. landdwellers are also very K-selected (few offspring with a relatively high chance of survival), maybe even more so than humans, because offspring who don't survive until adulthood just waste very limited space and resources.
this is completely separate from the seadwellers' own reproductive cycle. culturally, their pollination of landdwellers is in the same category as sex, because it involves the same sort of trust / social rituals / pleasure, but biologically it's a feeding behavior, and seadwellers have their own separate reproductive cycle, with two sexes and minor sexual dimorphism, like humans. landdwellers can also reproduce in landdweller-landdweller pairs, in a pinch.
i'm thinking seadwellers historically lived in relatively shallow seas (i.e. not deep seas), in coral reefs or kelp forests or something else with things to grab onto and make tools of. im imagining the landdwellers as roughly humanoid while the seadwellers have a mermaid-like body plan. the history of the two species has been deeply intertwined since before they've existed in their modern form, and there's been selection pressure to keep the two species able to understand and replicate each other's social communication, eventually including language. while there are many languages on the planet, they're not generally separated into "landdweller languages" and "seadweller languages", instead both the landdwellers and seadwellers in a particular area have shared a common tongue and other cultural aspects. in the modern world, there's the technology for landdwellers to have air-bubble enclaves in seadweller communities, and vice versa with water-filled seadweller structures on land.