I think you don't understand the whole part about controlling their body at the molecular level.
They're literally merged, unmerge, and then merge again in seconds. The brother who unmerged made it all the way past the kids, neji included, without them noticing. Twice.
They merge much quicker than anything Neji has reacted to.
You must be registered for see links
You must be registered for see links
You must be registered for see links
It's not like Kiba was like sniff sniff "he's right on top of me"
Neji's vision may be > Kiba's smell and even that's arguable but it's not so much better that he would notice something that Kiba genuinely had no idea was happening.
Because when Sakon/Ukon break down on a molecular level, they can't be seen/ smelled.
He had no idea they had merged until he came out of his body.
You must be registered for see links