TLDR: It was forbidden to kill Tuvix, even to save two others.

The Talmud does not allow a person to actively kill someone to save a life. 1/?

Inaction that leads to a death is preferred to action that leads to a death. This applies even with a large number of people. Because the value of each life is infinite, you can't do simple 2>1 math. Infinite is infinite.

In one example Rambam says that if a group of thieves ask for a group of Jews to hand over one person or else the thieves will kill everyone, the Jews may not hand over the individual.