How can there be only two Halachic witnesses at a large wedding?

Question

By what power or logic can the mesader kiddushin saying that two specific, designated people are the only witnesses to a wedding change the fact that all assembled have seen all that transpired at the wedding ceremony (i.e., witnessed the ceremony)?

This exclusion of witnesses is described on this site which says,
"Confirming the Witnesses
The mesader kidushin asks the witnesses if they are related to either the bride or groom or each other. He then asks the bride and groom if they want these and only these witnesses to be their witnesses."

Answer 1

The Ritva in Kiddushin 43a brings the custom of designating specific witnesses at a ceremony where there are invalid witnesses present, to get around the problem of עדות שבטלה מקצתה בטלה כולה (the dictum that any single witness from a group of witnesses who is found invalid invalidates the entire group).

מיהו כל היכא שיש באותו מעמד כשרים ופסולים או קרובים צדיך לייחד עידי הקידושין דאי לא כיון דאיכא עד פסול ביניהם עדות כלם בטלה

The questioner seems to have understood that by designating witnesses, you invalidate all the other witnesses. I believe this is incorrect; nobody can stop witnesses from seeing. The Ritva means that by designating specific witnesses, they are no longer automatically part of the same 'group', and therefore the invalid witnesses cannot invalidate the kosher witnesses.

[Accordingly, another two kosher witnesses may separate themselves from the rest of the participants, and designate themselves as another 'group'; they would also be Kosher witnesses. If something then invalidates the two witnesses who were chosen by the Mesader Kiddushin/Chosson, these two would still act as valid witnesses.]

Answer 2

There are two issues at play here.

1) Who actually saw the event.

2) Who are the members of the "official group" ("Kaht") of witnesses that not only saw the event, but also serve as a specific group capable of authenticating the event for a Court of Law.

Everyone present probably saw the event. They are all witnesses.

However, the mesader kiddushin is asking the bride and groom to choose a limited group (2) of people to join the secret club called the "kaht" of witnesses to the kidushin act.

There is no power or logic to exclude the people there from the general group called "witnesses".

However, before the event takes place, there is power to create a special group of witnesses that exist as a privately locked group of 2 members by themselves; without joining the rest of the room's group.

So now we have two groups of witnesses.

A) 98 guests who were standing around and saw the event.

B) 2 guests that also saw the event but were designated as a private group of witnesses.

Now those two may go to court and give testimony without belonging to the other group of 98. That's the power of designation.

This prevents a large random group from forming and showing up to court, when they contain possible relatives. This would then invalidate the entire group as Halachah demands that a group of witnesses are either kosher or not kosher as a group.

Answer 3

Other answers have explained the possible logic for the exclusion. I would like to point out that in fact this whole idea is only the view of the Ritva, and was not accepted by most Rishonim. In fact, the Chasam Sofer writes that even if one of the two 'designated' witnesses was found to be invalid, there is no need for new kiddushin as the other people watching do count (the full text is available here). In other words, there is no power or logic to what the mesader kiddushin says.