Must be two witnesses. But, the elders would be watching the person like a hawk.
I know of one experience where this happened. No judicial action was taken on either person. If I remember right the teen that confessed was judged as repentant and the non confessing teen couldn't be touched due to the two witness rule. The elders investigated to see if other teens knew they were alone together or if they had seen other instances of "brazen conduct" but no knew or would rat. They watched the teen for years just waiting for something to happen.
What a messed up religion.