My understanding is that they don't use the exact same structures (IN uses motor cortex, FS uses frontal cortex, VD uses... chakra cortex?) so they should be able to slot together. But there might indeed be overlap and issues that arise there.
Correct me if I'm wrong, but aren't the "this area of the brain deals with this task" just approximations/simplifications? I mean yes, damage to certain areas demonstrably leads to loss of certain functions but that doesn't mean there's no...err...subcontracting and cooperation between different brain areas.
I can't remember what different areas of the brain are supposed to do what, and wikipedia's presentation on what is what is surprisingly convoluted. (...and slightly inconsistent in terminology? This is why using a single book as a reference material is better than using a wiki. The books are usually edited for consistency so it's easier to get the big picture out of them without reading through everything three times.) So I'll just refer to what (ostensibly) similar brain functions IN as FS touch according what we've seen in the quest.
Both IN and FS deal with decision making. IN lets Hazou branch out the different (mostly apparent) options in combat at high speeds and pick the correct one, whereas FS enables Keiko to consider the options more in the abstract and add more in depth analysis on probabilities at the price of impaired creativity. These seem like things that come from overlapping brain areas.
Also note how after combat (see aftermath of Let The Bodies Hit The Floor) Hazou seemingly loses some of his personality and has a really hard time with social interactions...almost like Keiko when she starts starts dipping into the bloodline. Both of them also have attention issues soon after dipping into their bloodlines.
IN came from doujutsu and there's a strong visual component (see Hazou fighting and copying seals). I'm pretty sure that when Keiko dips into her bloodline there was weird visual feedback involved (can't remember where in the quest we see it for the first time) so that's probably another area where they link.
There was also other sort of sensory feedback (I think "icy" and "cold" were the words used) included with the Mori trance. Motor control, which IN is all about, and sensory feedback were pretty closely linked inside the brain if I remember correctly (pretty strong IF) so I think we have a one more function that the two bloodlines tamper with.
That's quite many conflict areas just inside the the brain and between two bloodlines. I'm pretty sure we will find more if add Wakahisa to things and consider rest of the nervous system starting from the spine and moving down. Then again, if we can reliably mess with the brain rest of the nervous system won't be too much of a hurdle. Dropping chakra on top of this might makes things even more complicated (or not, depending how the flesh/chakra interaction really works).
I'm thinking that messing with all that inside the human body will be really complicated if we don't want to end up with a lot of dead bodies (or brain-dead teammates). It might be easier to take the good bits and try to build some sort of external biomachinery for specific tasks. We could use IN for the control system, VD for routing and storing power (still need to get the power from somewhere...but meh) and FS for processing needs.
The problem is that adding the FS part will probably lead to eldricht horrors trying to invade the physical world through our new bioroids...but I'm sure that won't be as a bad as it sounds.
EDIT: Now that I wrote all that I get the feeling that someone made all these points already way back in the thread...Oh well, there's bound to be some repetition in a thread this long.
