What is the meaning of a two part poison?

Question

In Season three:

Londo poisons Rifa using a "two part poison"

What is the meaning of such a poison? Since the first is totally ineffective without the second half, why is poisoning someone in advance with the first half meaningful?

The analogy is a perfect encryption system - one time pad. You take a message M, and a secret key K, and then encrypt the message by xoring it with the key to get M⊕K. Neither K nor M⊕K leak any information alone, they're only meaningful if you have both parts.

So, is the threat that "I have poisoned you with one half of the poison" merely symbolic in this case? Why did this threat have the intended effect? If instead the threat was "do what I want or else I'll poison you", would that have worked as well?

Answer 1

The most common use of a two part poison is to bypass protections against poison.

Whether it is a device that scans for poisonous elements in food, or a personal taste tester, the two non-poisonous components are more likely to be ingested by the targeted victim.

In the event that the poisoner announces that they've administered the first half of a two part poison, it is likely a statement that "I managed it the first time, and I can do it again, so consider warning to have been served."

Answer 2

Beofett's answer is a good one. I would like to point out an additional, real-world, reason that poisons may be stored as two parts ("binary toxins").

An example of a binary toxin from real life is VX nerve gas. VX nerve gas is very, very poisonous. Very small amounts of this toxin can kill a human in 30 seconds!

Given the example of such an extremely toxic and fast-acting poison, it is useful to store the poison in a non-toxic form. The two ingredients which produce VX are kept apart from each other until immediately prior to using the VX. In this case, the binary nature of the toxin makes it safer for the user.

In the case of Londo poisoning Rifa, I think Beofett's answer is more relevant. In that type of fictional scenario, the pretext seems to be that Londo is able to bypass security by using a binary toxin. If Londo can identify two social functions, both of which he and Rifa, but no other person, will attend, then Londo can poison the punch at each function with the two halves of his binary compounds. Presumably, in the fictional context, these ingredients will not be recognized as poisons, allowing Londo to act without detection.