There may be spoilers in my answer but really anyone on this site has 99% chance already read the first book or at least one of them.
Unfortunately this question has never been addressed in the books so far written. It's an interesting question though.
I would have to believe because of the same principle that protects Richard from Kahlans touch that the second touch would have no effect even if it were by a more powerful confessor.
Richard being touched by another Confessor is sadly a difficult thing to judge. The principle was that he was so devoted to Kahlan that her Magic could not make him any more so. Thus it's basically as if he was already touched by her power and thus would be immune to others. This is open for major arguments though as there is no real way to say with any certainty.
Another question you may ask yourself is what would happen if Richard somehow fell out of love with Kahlan or cheated on her. Would he still be protected from her power then? I would say no he would no longer be immune.
There was an episode on the TV-show "Legend of the Seeker" when a confessor touched Richard. The confessor suggested that she could now mate with Richard (Because to save her kind (Confessors). So perhaps nothing will happend.
But as EvilLoynis said, this question has not been addressed in the books.
Sorry for bad grammar, English isn't my native language
It has been stated that if you are touched by a Confessors power whoever you are, all your previous emotions and the like are replaced by total devotion to the Confessor. Richards protection from Kahlans power is his love and total devotion to her. Because he would not be already devoted to this Confessor her power would destroy who he was, his devotion to Kahlan, and replace it with devotion to the other Confessor. These are just my thoughts on the matter.
All this would be just theorizing. The possibilities would include:
1) The new touch overrides the previous one, so the victim is utterly devoted to his new mistress with no loyalty remaining to the old until the new confessor dies.
2) This could quite possibly destroy the victim's mind. Two powerful magics tearing the mind's devotion into two directions. The magic is already powerful enough that the person will die on command if requested, so if you have two conradictory loves and sets of objectives the victim's mind could just not process it and die.
3) The show seemed to imply a touch would make the victim immune to a second touch which that girl confessor ordered Richard to do Kahlan. Not that the show has been consistent with the novels... or even itself.
Given the lack of Confessors it seems unlikely we'll enconter this short of TG doing another Magda Searus novel maybe.