The last time Alma preached in Ammonihah, he did it alone. The wisdom of the Lord's plan to have Amulek provide a second witness now becomes evident. The trap they set for Alma backfired because of Amulek's testimony, when Amulek had spoken these words the people began to be astonished, seeing there was more than one witness (Alma 10:12). The Lord hereby demonstrates that his wisdom is greater than the cunning of the devil (DC 10:43).
The tone of the question asked of Alma implies that the people understood that every word should be established at the mouth of two witnesses, or three witnesses (Deut 17:6), but they had no real intention of believing the testimony regardless of how many witnesses there were. This demonstrates that they were willfully rebellious, knowing the letter of the law but ignoring the Spirit.