Here is an all but equivalent example: you have two terminally ill patients - alive in a very real, indeed, literal sense.
They will be dead* in a month if you do not find a heart for one and a whole liver for the other.
If you kill another person and chop her up for heart and liver, you will save these patients.
You actually think killing a person to save these two patients is justified?
Or that watching the patients die (or slip into a coma) because you cannot save them (without killing another person) is murder?
*or they will slip into a coma - if you want to be medically unrealistic, as the star trek situation is.
Sorry to chime in here - especially since I don't think Janeway's decision was right - but your example is really *not* an equivalent to what happened in "Tuvix".
Most importantly, Tuvok and Neelix were dead by the time Tuvix came to life. There was no other way to revive them than to kill Tuvix. In your example, your two patients are still very much alive, and a lot can happen within a month, another donor could be found etc. So, no need to go to drastic measures and kill a person just for the off-chance that he/she's a suitable donor for *both* patients - let's not even mention the legal implications here. I'm a medical doctor, so forgive me but your example really lacks any similarity to Tuvix's situation.