I already made a similar question searching for possible features which can make this fictional specie biologically functional, so this question can be considered a second part of this. I would recomend read the previous question but anyways here a little description
Schistosoma is a flat worm specie in which the male is notorious bigger than the female, and this have a curvated chanel in which takes the female for give her protection, this causes a constant copulation while are together, they bond themselves by the presence of a suction cup. Females can go out from this chanel and find other males, but the males compete for evite this. With a little more of context, the answer of my previous question proposes traumatic insemination in the uterus with genital stabbing like a water bug and fusion of the circulatory and nervous system as an enhanced version of what a monkfish would do.
At that time I had the idea of the concept, but not about the characteristics and biological mechanisms that would make this work, so this is what I must build the general anatomy of this species, therefore I have some problems.
First to make this design work, How much difference in size should the male and female have and how much is realistically possible?
What possible anatomical characteristics should the male and female have for this mechanism to work?
With this I mean, in one of my design tries the females is like a little centauroid (more similar to a naga or "amphisbaenia centaur") while the male is giant hexapod gorilla.
Previously I wanted to take start this from a terrestrial specie, but now I am open to make this an alien or gentically created specie (my point is just applye the strange Schistosoma like feature).
And finally a little aclaration, maybe this could sound to other similar questions about sexual dimorfism and how much different male and female can be, but in this case is specifically to work as the described way.