This question is bugging me like forever...why did Kagero's poison not kill Tenzen? I know, the twin did survive.
But why...? After all, they shared the same body, same blood, same lymph. The poison should have killed him too!
Was there sth I missed?
...or did the mangaka simply want the main characters to finish off the Last Boss?

That parasite thing in tenzen's body kept him from dying. As long as his body is intact he's alright. The same thing happened in the movie Shinobi and he lived.

Poison got into Tenzen's body. Travelling with blood and lymphe (or simply by diffusion ), it should have got also to the twin. After all, twin's body was made from the same tissues as Tenzen's, so it had to have the same vulnerability to the poison. It should have died. Well, in the Shinobi movie Tenzen survived some wounds, but died ( just as his worms did ) from the poison

the thing that lives inside of Tenzen (what you call the twin) is impervious to Kagero's poison.