Logical solution to the poisoned cup problem from “The Princess Bride”

In the film The Princess Bride, Westley/The Dread Pirate Roberts played by Cary Elwes places two cups filled with wine in front of the kidnapper Vizzini, played by Wallace Shawn and challenges him to deduce which of them has been poisoned with Iocaine. In the end it turns out that both cups were poisoned but that Westley had built up a tolerance to Iocaine poison.

My solution:

It is obvious from the very start of the challenge that both cups are poisoned.

Why?

Westley could not afford to poison just one because he could not take the risk that Vizzini would get lucky and pick the one without poison in it. Westley, however, clever he was could never predict which cup Vizzini was going to pick and yet he wanted to poison Vizzini and so he had no option but to poison both of them.