How would you treat a paladin in the case of being combined with anybody that would not follow the paladin code? For example, say a paladin was merged with a serial killer. Is it an insta-fall? There could be some great potential for roleplay there, but also plenty of potential for unnecessary angst and conflict.
I think the key words are "willingly commits an evil act." As long as the paladin always acts righteously when in control, falling wouldn't be an issue. The powers that be can look into the paladins soul and discern intent. With that said, however, I'm unlikely to accept a serial killer for just this reason. If you play a paladin, prepare for some angst. Of course, playing a paladin ALWAYS involves angst, so no real loss there.
Edit: Also keep in mind the paladin has already chosen to adventure with this person, so how evil are they likely to be?