My previous question was how to classify a domain (now superdomain) of duocellular life named Duotorusa.
Anyone who has taken high school biology know that all life on earth are divided into three different domains: Bacteria, Archaea, and Eukaryota. On a planet in the outer habitable zone of a K-type star with approximately 1.5 Earth oceans of water, there is another domain: Duotorusa.
Given the nice color-coding of the scanning-microscope Duotorusa unit (just kidding, this is an image borrowed from TeX.SE), I will call the cell units red and blue respectively. These are my current ideas to their functions, those can be changed in the answers if necessary to increase the evolutionary advantage.
The red cell helps power the organism. Propulsion (cilia or flagella) on the red cell moves the organism, and it also provides (photosynthesizes/finds) food, which it eats. It is a prokaryote, which only carries the DNA/RNA necessary for finding/obtaining food and moving based on where the food is.
The blue cell is the reproductive center of the organism. It has a strong, cellulose backbone that provides structure to the unit, as well as a nucleus that stores DNA necessary for reproduction, regeneration of the red unit, which is expendable when harsh conditions occur and the blue cell must hibernate (oh yeah, it can do that too). The blue cell, when conditions allow (lots of food/space), produces a new Duotorusa blue unit which it expels with some cytotoxic waste (just to make sure the 'child' is safe).
My question is, what makes a Duotorusa with one red unit more advantageous than one with like, 100 units, or one that can have a near-infinite amount of red units? Why do only two units have to stick together?