Anytime we're trying to testify that somebody owes money to somebody else, that they have to pay someone else, we require two witnesses to testify. And when it comes to the, we say that the testimony could apply only when there are at least two witnesses who would technically be able to testify for the person making the claim. Here, the Mishnah is going to tell us about a situation where two or more witnesses deny the testimony and become obligated to bring a korban (offering).
The Mishnah says that if two witnesses together deny knowing testimony about somebody that would have been needed, they're both obligated to bring a korban. If they make their denials one after the other, the first witness is obligated to bring a korban; the second witness is obligated to bring a korban, but the second witness is not obligated. The first witness is obligated because when he denied knowing the information, it was still possible for the two witnesses who saw and knew the information to come forward and give testimony. But once the first witness became pasul (disqualified), he gave his testimony; he had already claimed he didn't know anything. Then there's only one witness left: the second witness. So if he denies it, he's not technically really causing any loss to the person asking for his testimony, because he's the only witness left. He's not going to really have an effect anyway, so we say he would not have to bring a korban.
One of the witnesses denied knowing the testimony, and the other one admitted that he knew the testimony. The one who denies knowing the testimony is obligated to bring a korban. It doesn't matter who spoke first. Even if the first witness was the one who admitted that he knew something and the second one said he didn't, he's now preventing testimony regardless of the order. He's preventing testimony from being given and from helping out this person. So regardless of when he spoke, we say that the person who denies it is obligated to bring a korban. The other witness who's willing to testify is the one who denied knowing the testimony and therefore caused a loss to the person asking for the testimony, so he is obligated to bring that korban.
If there were two separate sets of witnesses who were asked by somebody to testify on his behalf, the first group denied knowing the testimony, and afterwards the second set of witnesses denied knowing the testimony. Both sets are obligated to bring korbanot. Why? Because the testimony could have been established through either set of witnesses in this case. Even after the first set of witnesses, who could have had an effect here, denied their testimony, we say they have to bring a korban because they could have helped. The second set could still help at this point because there are two of them and they hadn't said anything yet. So when they deny it and refuse to help, we say that they too are denying testimony and causing the person to lose money, and therefore they are also penalized. They have to bring a korban as well.