Interesting. I never saw it happen, all though I heard of it. (I was an elder for over 20 years).
The Two Witness Rule would absolutely apply. The denier would probably have to offer some explanation as to why the other person would "confess to something that never happened," but they could cite the case of the Amalekite that claimed he killed Saul as Biblical precedent for people lying about things they've done for selfish gain.
The motive in a situation like this would have to be something like some have mentioned above: obsession or insane desire to cause trouble.
If the denier stuck to their guns and there was no other witness, the elders would have no WT sanctioned policy upon which to proceed (notice I did NOT say scriptural).
All that being said, no one should ever admit anything to the elders. They are not your friends and what ever you do in private is none of their business! NONE!