Anatomical considerations
If you’re looking at quite deep integration (fused circulation/nervous system) then you are constrained by the fact that the brain issuing motor orders must be somewhat proportionate to the body executing them. Assuming you don’t just want a big female head on a male body, you have a couple of ideas you could consider:
* Not all animals use an explicit spatial map-like representation for their motor control (a somatotopic map); the octopus delegates this fine topography to large neural bundles in each arm, while the central brain only issues high-order commands. This means that the brain can remain relatively small (only 1/3 of an octopus’ neurons are in the central “brain”) and somewhat detached from the practicalities of how movement is mechanically achieved. Your female may have a similar role, with the bulk of pattern-to-muscle details entrusted to the male, so that her brain would not need to scale with the male’s body size
* You may not need or want directly fused nervous systems anyway. The way in which the female directs the male could be by compulsion, guidance or persuasion. Imagine that the pair bonding process releases powerful hormones that make the male deeply invested in keeping the female happy, or satisfying her wishes. This relationship might be closer to the commitment of social insects to the hive: a forager bee isn’t following commands from the queen, she just really really wants to collect pollen to take home.
But why does the male not just think for himself? One option is that stupidity is used by this species as a sexually selected trait. 335
Putting yourself (and your genes) at unnecessary risk is pretty much the point of the male sex: if you can survive despite being this dumb, you must be really strong, fast, resilient etc. As long as the hobble trait (stupidity) does not manifest in females, it’s a perfect tool of hyperselection. So imagine a teenage phase in which unpaired males just out-idiot each other by swallowing Tide pods and taking selfies next to predators and what have you, which for some reason the ladies find irresistible. After a boy finds a girl, she keeps him straight with good advice and potentially a bit of octopodean mind control.
Reproduction
I can only respect an answer that brings up hypodermic traumatic insemination, but I really don’t think it makes any sense in this scenario. This is a last-resort solution in some promiscuous species with high male-male competition and very adversarial sexual selection practices, and is detrimental to females (as you might imagine), which fits poorly with a symbiotic relationship with stable pairs. The most obvious solution seems to me just boring old external fertilisation, where females release eggs after pair bonding and males inseminate them on their own time after collecting them. Seahorses had been brought up and are probably the best example.
If creating eggs makes the female really particularly vulnerable, this may explain why they developed a relationship so dependent on the male, and why they would evolve a mechanism to select a male who commits physically to protecting them through the next “pre-pregnancy”; the male, in turn, by bonding to the female, ensures that the egg she finally produces is available to be inseminated by him and not a random other male she fancies. 1008