According to the Thirteenth Doctor, binary demi-species were rare. Individuals from such species were symbiotic hybrids of two living forms, which could meld or unmeld from one another at will; one was usually more intelligent and the other more dangerous and animalistic. When both their forms were in a single body, they had two pulses at once.
The Grand Serpent was a member of a binary demi-species, with the ability to project a serpent-like creature with which he was symbiotically bonded with out of himself. (TV: The Vanquishers)