I doubt that the witnessing judges would have to recuse themselves. The fact that they know (or at least think they know) what actually happened in that room would be a plus, not a minus, and no reason for recusing themselves.
Politically, the sitting judges will always have a stake in the matter, so that’s not a reason to recuse. Directly witnessing? I tend to doubt it, but I could be wrong.
Time to clean house. I say, let’s get on with it.
I don’t know about Wisconsin, but the Federal standards require recusal “where he has a personal bias or prejudice concerning a party, or personal knowledge of disputed evidentiary facts concerning the proceeding.” The Supreme Court Justices certainly have knowledge of what will be disputed evidentiary facts, disputed amongst the Justices themselves.