Janusian

Janusians are a prominent alien species on the Great Ship. They almost always appear as couples.

Biology
Janusians are born as individuals. The tiny male burrows into its mate's back, permanently joining them. Like most technologically advanced species, Janusians have embraced technology that makes them immortal. After being joined for a vast length of time, the two can grow into a single entity.

In The Realms of Water, it's stated that the parasitic body is male only half of the time, and the larger partner is often not the dominant one.