No canon answer.
From the books, we can learn that:
- A Confessor's touch removes something of a person's self. As the magic takes it away (to the Underworld), the person touched is an empty shell, totally devoted to the Confessor in a form of unconditional love*.
- A Confessor's touch does not go away when she dies, a person touched may not survive such a loss*.
- Effects of the touch are diminished when the subject is transformed into an animal by a Wizard**.
*Kahlan's father, a mate to her mother, the previous Mother Confessor, was so devoted to her, that he died on the spot when he found out that she was terminally ill.
The Stone Of Tears, Chapter 11
“My father was mate to a Confessor. That was all that was in him. When my mother was
dying of a terrible wasting illness, he was in a constant state of panic. One day the wizard
and the healer who had been tending her came to us and said there was nothing more they
could do, that the spirits would soon take her to be with them, that she would soon pass
“With a wail of anguish like none I have ever heard, my father clutched his chest and fell to
the floor, dead.”
This is contradicted by the TV series, where death of a Confessor releases their victim.
** Brophy, a suspect to murder found innocent by Kahlan, is still following her, after being transformed into a wolf by Giller, Kahlan's acccompanying Wizard. However, he lives a normal wolf life.
Wizards First Rule, Chapter 38
[Brophy:] “Mistress, when I was changed to the wolf, it removed most of my need for you, my need to please you. Yet I would still die for you.
Knowing that, we can deduce that a person already confessed has nothing for the second confessor to take away. It is possible that the second touch will either kill them, or drive them mad. However, it may cause no effect:
At the end of Wizard's First Rule,
Richard, being ultimately devoted to Kahlan through love, survived her touch intact.
UPD: Another consideration is that a second confessor has no point of confessing that person. As all Confessors belong to the same group, led by The Mother Confessor, they all pursue the same goal. If one Confessor has touched a person, she has probably solved the case and proved that he/she is guilty (typically such a person dies on the spot after confession) or innocent (then he/she is turned into an animal of choice by the Confessor's accompanying Wizard). If a Confessor uses her power on a person for self-defense, and decides to keep them alive, she would keep them close (eg. to protect her, like Kahlan used Orsk) - the person confessed will be eager to do so, unless she sends them on some suicide mission (Kahlan sent captured Order troops back to try to assasinate Jagang). Finally, a Confessor touches a man to be her mate, to bear a daughter Confessor.
Conclusion: a Confessor is unlikely to meet a person already touched by another Confessor (at least without that Confessor nearby), and is unlikely to have a reason to confess that person again.